Fuse Saver

This circuit will be
particularly useful to those hobbyists who use a ‘breadboard’ to try out
ideas and who also use a simple ‘home-made’ DC power supply consisting
of a transformer, rectifier, smoothing capacitor and protective fuse,
that is, one without over current protection! In this circuit, the
detecting element is resistor R6. Under normal conditions, its voltage
drop is not high enough to switch on transistor T1.

The value of R6 can be altered to give a different cut-off current,
as determined by Ohm’s Law, if required. When a short circuit occurs in
the load, the voltage rises rapidly and T1 starts to conduct. This draws
in the relay, switching its contacts, which cuts off power to the
external circuit, and instead powers the relay coil directly, latching
it in this second state. The circuit remains in this state until the
primary power supply is switched off.

Capacitors C1 and C2 hold enough charge (via D3, D4 and D6, which
prevent the charge from being lost to the rest of the circuit, whichever
state it is in) to keep T1 switched on and power the relay while it
switches over, and R2 and R4 provide slow discharge paths. LEDs
D1 (red) and D5 (green) indicate what state the circuit is in. Inductor
L1 slows the inrush of current when the circuit is switched on, which
would otherwise cut off the circuit immediately.

Fuse Saver Circuit

Fuse Saver Circuit Diagram

D2 and D7 provide the usual back-emf protection across the coils. In
use, the input of the circuit is connected to the main
transformer-rectifier-capacitor-fuse power supply via K1, and the output
is connected to the (experimental) load via K2. Note that the input
voltage must be a floating supply if Vout– is grounded via the load, as
Vin– and Vout– must not be connected together. Some consideration needs
to be given to a number of components.

First, the choice of relay Re1. For the prototype, this was obtained
from Maplin, part number YX97F. This is has a coil resistance of 320 ?,
which with R1 forms the collector load for T1. Its allowed pull-in
voltage range is nominally 9 V to 19 V, which limits the input power
supply voltage to between around 10 V to 30 V (DC only). R1 could be
replaced by a wire link for operation at input voltages below 10 V, or
increased in value, as determined by either the application of Ohm’s Law
once more or trial and error, for an input voltage above 30 V.

Coil L1 was obtained from Farnell, part number 581-240. Finally, the
protective fuse for the input power supply should be a ‘slow-blow’
type; ‘fast’ fuses will rupture before the relay has time to switch.
Also note that this device is meant to save fuses, not replace them. A
mains transformer must always be fused if it is not designed to run
safely, i.e., without presenting a fire hazard, even if its output has a
continuous short-circuit fault.

Author: David Clark – Copyright: Elektor Electronics Magazine


Intelligent Electronic Lock

This intelligent
electronic lock circuit is built using transistors only. To open this
electronic lock, one has to press tactile switches S1 through S4
sequentially. For deception you may annotate these switches with
different numbers on the control panel/keypad. For example, if you want
to use ten switches on the keypad marked ‘0’ through ‘9’, use any four
arbitrary numbers out of these for switches S1 through S4, and the
remaining six numbers may be annotated on the leftover six switches,
which may be wired in parallel to disable switch S6 (shown in the
figure). When four password digits in ‘0’ through ‘9’ are mixed with the
remaining six digits connected across disable switch terminals,
energisation of relay RL1 by unauthorised person is prevented.


Circuit diagram

For authorised persons, a 4-digit password number is easy to
remember. To energise relay RL1, one has to press switches S1 through S4
sequentially within six seconds, making sure that each of the switch is
kept depressed for a duration of 0.75 second to 1.25 seconds. The relay
will not operate if ‘on’ time duration of each tactile switch (S1
through S4) is less than 0.75 second or more than 1.25 seconds. This
would amount to rejection of the code. A special feature of this circuit
is that pressing of any switch wired across disable switch (S6) will
lead to disabling of the whole electronic lock circuit for about one

Even if one enters the correct 4-digit password number within one
minute after a ‘disable’ operation, relay RL1 won’t get energised. So if
any unauthorised person keeps trying different permutations of numbers
in quick successions for energisation of relay RL1, he is not likely to
succeed. To that extent, this electronic lock circuit is fool-proof.
This electronic lock circuit comprises disabling, sequential switching,
and relay latch-up sections. The disabling section comprises zener diode
ZD5 and transistors T1 and T2. Its function is to cut off positive
supply to sequential switching and relay latch-up sections for one
minute when disable switch S6 (or any other switch shunted across its
terminal) is momentarily pressed.

During idle state, capacitor C1 is in discharged condition and the
voltage across it is less than 4.7 volts. Thus zener diode ZD5 and
transistor T1 are in non-conduction state. As a result, the collector
voltage of transistor T1 is sufficiently high to forward bias transistor
T2. Consequently, +12V is extended to sequential switching and relay
latch-up sections. When disable switch is momentarily depressed,
capacitor C1 charges up through resistor R1 and the voltage available
across C1 becomes greater than 4.7 volts. Thus zener diode ZD5 and
transistor T1 start conducting and the collector voltage of transistor
T1 is pulled low. As a result, transistor T2 stops conducting and thus
cuts off positive supply voltage to sequential switching and relay
latch-up sections.

Thereafter, capacitor C1 starts discharging slowly through zener
diode D1 and transistor T1. It takes approximately one minute to
discharge to a sufficiently low level to cut-off transistor T1, and
switch on transistor T2, for resuming supply to sequential switching and
relay latch-up sections; and until then the circuit does not accept any
code. The sequential switching section comprises transistors T3 through
T5, zener diodes ZD1 through ZD3, tactile switches S1 through S4, and
timing capacitors C2 through C4. In this three-stage electronic switch,
the three transistors are connected in series to extend positive voltage
available at the emitter of transistor T2 to the relay latch-up circuit
for energising relay RL1.

When tactile switches S1 through S3 are activated, timing capacitors
C2, C3, and C4 are charged through resistors R3, R5, and R7,
respectively. Timing capacitor C2 is discharged through resistor R4,
zener diode ZD1, and transistor T3; timing capacitor C3 through resistor
R6, zener diode ZD2, and transistor T4; and timing capacitor C4 through
zener diode ZD3 and transistor T5 only. The individual timing
capacitors are chosen in such a way that the time taken to discharge
capacitor C2 below 4.7 volts is 6 seconds, 3 seconds for C3, and 1.5
seconds for C4. Thus while activating tactile switches S1 through S3
sequentially, transistor T3 will be in conduction for 6 seconds,
transistor T4 for 3 seconds, and transistor T5 for 1.5 seconds.

The positive voltage from the emitter of transistor T2 is extended
to tactile switch S4 only for 1.5 seconds. Thus one has to activate S4
tactile switch within 1.5 seconds to energise relay RL1. The minimum
time required to keep switch S4 depressed is around 1 second. For
sequential switching transistors T3 through T5, the minimum time for
which the corresponding switches (S1 through S3) are to be kept
depressed is 0.75 seconds to 1.25 seconds. If one operates these
switches for less than 0.75 seconds, timing capacitors C2 through C4 may
not get charged sufficiently. As a consequence, these capacitors will
discharge earlier and any one of transistors T3 through T5 may fail to
conduct before activating tactile switch S4.

Thus sequential switching of the three transistors will not be
achieved and hence it will not be possible to energise relay RL1 in such
a situation. A similar situation arises if one keeps each of the
mentioned tactile switches de-pressed for more than 1.5 seconds. When
the total time taken to activate switches S1 through S4 is greater than
six seconds, transistor T3 stops conducting due to time lapse.
Sequential switching is thus not achieved and it is not possible to
energise relay RL1. The latch-up relay circuit is built around
transistors T6 through T8, zener diode ZD4, and capacitor C5. In idle
state, with relay RL1 in de-energised condition, capacitor C5 is in
discharged condition and zener diode ZD4 and transistors T7, T8, and T6
in non-conduction state.

However, on correct operation of sequential switches S1 through S4,
capacitor C5 is charged through resistor R9 and the voltage across it
rises above 4.7 volts. Now zener diode ZD4 as well as transistors T7,
T8, and T6 start conducting and relay RL1 is energised. Due to
conduction of transistor T6, capacitor C5 remains in charged condition
and the relay is in continuously energised condition. Now if you
activate reset switch S5 momentarily, capacitor C5 is immediately
discharged through resistor R8 and the voltage across it falls below 4.7
volts. Thus zener diode ZD4 and transistors T7, T8, and T6 stop
conducting again and relay RL1 de-energises.


Central Locking Interface

Some cheap car alarms do
not have a connection for the central locking system. However, in most
it should be possible to find a point in the alarm circuit which is high
when the alarm is activated and low when it is off. This signal can
then be used to drive this relay circuit to operate the central locking
system. The interface circuit converts each toggle of the alarm signal
to a brief pulse to operate the two relays which then are then connected
in parallel with appropriate contacts on the master solenoid in the
central locking system.

Central Locking Interface Circuit

Central Locking Interface Circuit Diagram

Author: Frank Keller – Copyright: Silicon Chip


High Low Voltage Cutout Without Timer

This inexpensive circuit
can be connected to an air-conditioner/fridge or to any other
sophisticated electrical appliance for its protection. Generally, costly
voltage stabilizers are used with such appliances for maintaining
constant AC voltage. However, due to fluctuations in AC mains supply, a
regular ‘click’ sound in the relays is heard. The frequent
energisation/de-energisation of the relays leads to electrical noise and
shortening of the life of electrical appliances and the
relay/stabilizer itself. The costly yet fault-prone stabiliser may be
replaced by this inexpensive high-low cutout circuit with timer.

The circuit is so designed that relay RL1 gets energised when the
mains voltage is above 270V. This causes resistor R8 to be inserted in
series with the load and thereby dropping most of the voltage across it
and limiting the current through the appliance to a very low value. If
the input AC mains is less than 180 volts or so, the low-voltage cut-off
circuit interrupts the supply to the electrical appliance due to
energisation of relay RL2. After a preset time delay of one minute
(adjustable), it automatically tries again. If the input AC mains supply
is still low, the power to the appliance is again interrupted for
another one minute, and so on, until the mains supply comes within
limits (>180V AC).


Circuit diagram

The AC mains supply is resumed to appliance only when it is above
the lower limit. When the input AC mains increases beyond 270 volts,
preset VR1 is adjusted such that transistor T1 conducts and relay RL1
energises and resistance R8 gets connected in series with the electrical
appliance. This 10-kilo-ohm, 20W resistor produces a voltage drop of
approximately 200V, with the fridge as load. The value and wattage of
resistor R8 may be suitably chosen according to the electrical appliance
to be used. It is practically observed that after continuous use, the
value of resistor R8 changes with time, due to heating. So adjustment of
preset VR1 is needed two to three times in the beginning.

But once it attains a constant value, no further adjustment is
required. This is the only adjustment required in the beginning, which
is done using a variac. Further, the base voltage of transistor T2 is
adjusted with the help of preset VR2 so that it conducts up to the lower
limit of the input supply and cuts off when the input supply is less
than this limit (say, 180V). As a result, transistor T3 remains cut off
(with its collector remaining high) until the mains supply falls below
the lower limit, causing its collector voltage to fall. The collector of
transistor T3 is connected to the trigger point (pin 2) of IC1. When
the input is more than the lower limit, pin 2 of IC1 is nearly at +Vcc.

In this condition the output of IC1 is low, relay RL2 is
de-energised and power is supplied to the appliance through the N/C
terminals of relay RL2. If the mains supply is less than the lower
limit, pin 2 of IC1 becomes momentarily low (nearly ground potential)
and thus the output of IC1 changes state from ‘low’ to ‘high’, resulting
in energisation of relay RL2. As a result, power to the load/appliance
is cut off. Now, capacitor C2 starts charging through resistor R6 and
preset VR3. When the capacitor charges to (2/3)Vcc, IC1 changes state
from ‘high’ to ‘low’. The value of preset VR3 may be so adjusted that it
takes about one minute (or as desired) to charge capacitor C1 to

Relay is now de-energised and the power is supplied to the appliance
if the mains supply voltage has risen above the lower cut-off limit,
otherwise the next cycle repeats automatically. One additional advantage
of this circuit is that both relays are de-energised when the input AC
mains voltage lies within the specified limit and the normal supply is
extended to the appliance via the N/C contacts of both relays.

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Switch Timer For Bathroom Light

This 9-minute timer
switch can be used to control the light in a toilet or bathroom. The
timer is started by pushing S1 and stopped by pushing S1 again. If you
forget to turn it off, the controlled light will go off after nine
minutes. If you need the light on continuously non-stop, you need to
press S1 (turn on) and then S2 (cancellation of timer) within 9 minutes
and in this case the light will be on until you switch it off with S1.
IC1 is a is 4013 dual flip-flop. Flip flop IC1a is toggled on and off by
switch S1 and it controls the relay which is switched by FET
Q2. IC1a controls IC1b which is connected as an RS flipflop to enable
or disable IC2, a 4060 oscillator/divider. This has its timing interval
set by the components at its pins 9, 10 & 11. The relay should have
250VAC mains-rated contacts and these are connected in parallel with an
existing wall switch.

Switch Timer For Bathroom Light

Switch Timer For Bathroom Light

Author: Rasim Kucalovic
Copyright: Silicon Chip Electronics

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Relay Toggle Switch

I designed this relay circuit to function as a DPDT
toggle which is controlled by a momentary switch. I strive to keep my
circuits simple with little or no integration (555’s, transistors, etc).
The circuit is shown active or “on” mode.

Half of RL1 and RL2 manipulate the switching and the other is
connected to an application. Relays are 200 ohms above ground and at one
point are referenced to positive that turns them off.


RL1 (which is off) applies plus voltage from its armature and
latches RL2 “on”. The application terminals are set to [A]. The
condition changes when S1 is activated, voltage is applied to RL2
latching RL1 “on” releasing S1 turns RL2 “off”. RL2’s armature is then
directed to R1. Terminals are set to [B].

When S1 is pressed again, the relays negative side are referenced to
positive, RL1 turns “off” (there’s no current flow). RL2 turns “on”
when S1 is released, terminals are set to [A]. There is slight lag
between relays depending on how long S1 is held.

Relay Toggle Switch Circuit

Relay Toggle Switch Circuit Diagram


If different relays are used, adjustment of R1’s value may be required. For example, OEG
relays (12vdc, 270 ohm coil) need R1 at 60 – 70 ohms. The prime
motivation for this design was to avoid using toggle switches for my
audio control panel. Another plus, it can be controlled from a remote
transmitted pulse. Relays are available at allelectronics.com

Author: Roland Segers

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Momentary Switch Teamed With Latching Relay

This circuit allows an SPST momentary pushbutton to act as a push-on push-off switch, using a DPDT
latching (bi-stable) relay. It was originally intended to allow a
single pushbutton switch on the dash of a vintage car to provide a
latched function. The relay only draws current when it is being
switched. At other times, the only current drain on the 12V supply is
the leakage current of one 22µF capacitor, which is very low. It works
as follows.

Assume that initially the latching relay is in the reset state, with
pins 4 and 6 connected together. In this state, C2 charges up to +12V
via 2.2kO resistor R2 while capacitor C1 remains discharged as it is not
connected to the 12V supply. If S1 is pressed, C2 discharges via the
relay’s “set” coil, diode D2 and S1. This switches the relay into its
set position, connecting pins 4 and 8. C1 then begins to charge via R1.
While S1 is being held down, the relay does not return to the reset
position because the current supplied via R1 is insufficient for the
coil to latch the armature. As soon as S1 is released, current no longer
flows though the coil so C1 can finish charging, ready for the next
button press.

Momentary Switch Circuit Teamed With Latching Relay

Momentary Switch Circuit Diagram Teamed With Latching Relay

Once the relay has switched and C1 has finished charging, pressing
S1 again causes the relay to switch back to the reset state via the same
process. The unused set of relay contacts can be used as an SPST or SPDT switch. The circuit as shown has been tested with the Jaycar SY4060 relay. It will work with other DPDT
twin-coil latching relays but the resistor and capacitor values may
need to be adjusted to suit. Relays with lower resistance coils will
need larger value capacitors and smaller value resistors.

Author: Merv Thomas – Copyright: Silicon Chip Electronics Magazine


Pushbutton Relay Selector

This circuit was
designed for use in a hifi showroom, where a choice of speakers could be
connected to a stereo amplifier for comparative purposes. It could be
used for other similar applications where just one of an array of
devices needs to be selected at any one time. A bank of mechanically
interlocked DPDT pushbutton switches is the
simplest way to perform this kind of selection but these switches aren’t
readily available nowadays and are quite expensive. This simple circuit
performs exactly the same job. It can be configured with any number of
outputs between two and nine, simply by adding pushbutton switches and
relay driver circuits to the currently unused outputs of IC2 (O5-O9).

Gate IC1a is connected as a relax-ation oscillator which runs at
about 20kHz. Pulses from the oscillator are fed to IC1b, where they are
gated with a control signal from IC1c. The result is inverted by IC1d
and fed into the clock input (CP0) of IC2. Initially, we assume that the
reset switch (S1) has been pressed, which forces a logic high at the O0
output (pin 3) of IC2 and logic lows at all other outputs (O1-O9). As
the relay driver transistors (Q1-Q4) are switched by these outputs, none
of the relays will be energised after a reset and none of the load
devices (speakers, etc) will be selected. Now consider what happens if
you press one of the selector switches (S2-S5, etc). For example,
pressing S5 connects the O4 output (pin 10) of IC2 to the input (pin 9)
of IC1c, pulling it low.

Circuit diagram:

Pushbutton Relay Selector Circuit

Pushbutton Relay Selector Circuit Diagram

This causes the output (pin 10) to go high, which in turn pulls the
input of IC1b (pin 5) high and allows clock pulses to pass through to
decade counter IC2. The 4017B counts up until a high level appears at
its O4 output. This high signal is fed via S5 to pin 9 of NAND
gate IC1c, which causes its output (pin 10) to go low. This low signal
also appears on pin 5 of IC1b, which is then inhibited from passing
further clock pulses on its other input (pin 6) through to its output
(pin 4), thus halting the counter. So, the counter runs just long enough
to make the output connected to the switch that is pressed go high.
This sequence repeats regardless of which selector switch you press, so
the circuit functions as an electronic interlock system.

Each relay driver circuit is a 2N7000 FET
switch with its gate driven from one output of IC2 via a 100W resistor.
The relay coil is connected from the drain to the +12V supply rail, with
a reverse diode spike suppressor across each coil. If you want visual
indication of the selected output, an optional indicator LED
and series resistor can be connected across each relay coil, as shown.
For selecting pairs of stereo speakers, we’d suggest the use of relays
like the Jaycar SY-4052. These operate from 12V and have DPDT
contacts rated for 5A. Note that although four selector switches are
shown in the circuit, only two relay drivers are shown because of
limited space. For a 4-way selector, identical relay drivers would be
driven from the O2 and O3 outputs of IC2.

Author: Jim Rowe – Copyright: Silicon Chip Electronics


Reducing Relay Power Consumption

Relays are often used as
electrically controlled switches. Unlike transistors, their switch
contacts are electrically isolated from the control input. On the other
hand, the power dissipation in a relay coil may be unattractive for
battery-operated applications. Adding an analogue switch lowers the
dissipation, allowing the relay to operate at a lower voltage. The
circuit diagram shows the principle. Power consumed by the relay coil
equals V2/RCOIL. The circuit lowers this dissipation (after actuation)
by applying less than the normal operating voltage of 5 V. Note that the
voltage required to turn a relay on (pickup voltage)is usually greater
than that to keep it on (dropout voltage).

In this respect the relay shown has specifications of 3.5 and 1.5 V
respectively, yet the circuit allows it to operate from an intermediate
supply voltage of 2.5 V. Table 1 compares the relay’s power dissipation
with fixed operating voltages across it, and with the circuit shown here
in place. The power savings are significant. When SW1 is closed, current
flows through the relay coil, and C1 and C2 begin to charge. The relay
remains inactive because the supply voltage is less than its pickup
voltage. The RC time constants are such that C1 charges almost
completely before the voltage across C2 reaches the logic threshold of
the analogue switch inside the MAX4624 IC.

When C2 reaches that threshold, the on-chip switch connects C1 in
series with the 2.5 V supply and the relay coil. This action causes the
relay to be turned on because its coil voltage is then raised to 5 V,
i.e., twice the supply voltage. As C1 discharges through the coil, the
coil voltage drops back to 2.5 V minus the drop across D1. However, the
relay remains on because the resultant voltage is still above the
dropout level (1.5 V). Component values for this circuit depend on the
relay characteristics and the supply voltage. The value of R1, which
protects the analogue switch from the initial current surge through C1,
should be sufficiently small to allow C1 to charge rapidly, but large
enough to prevent the surge current from exceeding the specified peak
current for the analogue switch.

The switch’s peak current (U1) is 400 mA, and the peak surge current is IPEAK = (VIN – VD1) / R1 + RON) where RON
is the on-resistance of the analogue switch (typically 1.2 Ω). The
value of C1 will depend on the relay characteristics and on the
difference between VIN and the pickup voltage.
Relays that need more turn-on time requires larger values for C1. The
values for R2 and C2 are selected to allow C1 to charge almost
completely before C2’s voltage reaches the logic threshold of the
analogue switch. In this case, the time constant R2C2 is about seven
times C1(R1 + RON). Larger time constants
increase the delay between switch closure and relay activation. The
switches in the MAX4624 are described as ‘guaranteed break before make’.
The opposite function, ‘make-before break’ is available from the
MAX4625. The full datasheets of these interesting ICs may be found at http://pdfserv.maxim-ic.com/arpdf/MAX4624-MAX4625.pdf


Electronic Fuse Employs A Relay

While many power
supplies can be set to limit their output current to a defined level, to
protect the circuit they are powering, no such protection is available
if you are powering a circuit from a battery. If a fault develops, the
circuit can blow before you have a chance to disconnect it. Of course,
you can fit a fuse in series with the supply line to the circuit under
test but it will blow if a fault develops. Or perhaps it won’t blow
sufficiently quickly to protect the circuit. And repeatedly having to
replace fuses becomes a nuisance as well.


Circuit diagram

The alternative is to use an electronic fuse. This circuit uses a
relay to make and break the circuit. The current drain of the circuit
under test is monitored by a 1O 2W resistor which is placed in series
with the supply line. The voltage across this 1O resistor is monitored
by op amp IC1a which has an adjustable gain of between 11 and 16, as set
by trimpot VR1. The resultant DC voltage from pin 1 of IC1a is fed to
pin 5 of IC1b which is configured as a comparator. Trimpot VR2 provides
an adjustable voltage reference to pin 6 of IC1b and this is compared
with the amplified signal from IC1a.

If IC1b’s threshold is exceeded, its pin 7 goes high and this is fed
to Schmitt trigger inverter IC2a which then “sets” the RS flipflop
comprising gates IC2c & IC2d. Pin 11 of IC2d then goes high to turn
on transistor Q2 and LED1 while pin 4 of IC2b also goes high to turn on
Q1 and the relay which then disconnects the load. The circuit stays in
this state until the RS flipflop is reset by pushing switch S1.
Capacitor Cx, across the feedback resistance of IC1a, is used to
simulate a slow-blow or fast-blow fuse and can be selected by trial and
error. Changing the gain of IC1a or the value of the sensing resistor
changes the fuse rating of the circuit.

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