Circuit Project: A Handy Pen Torch

This easy to construct
“Handy pen torch” electronic circuit and low component count, uses two
power white LEDs for lighting. Low volt (4.8V
dc) supply available from the built in rechargeable Ni-Cd battery pack
is first converted into two channel (independent) constant current
sources by two pieces of the renowned precision adjustable shunt
regulator chip LM334 (IC1 and IC2). Around 25mA at 3.6 volt dc is
available at the output of these ICs.

This regulated dc supply is used to drive two power white LEDs
D4 and D6. Resistors R3 and R5 limits the output current (and hence the
light output) of IC1 and IC2 circuits respectively. Besides these
components, one red color LED (D2) is included
in the main circuit which works as a battery charging supply input
indicator. Resistor R1 limits the operating current of this LED.

Pen Torch Electronic Circuit Schematic

Diode D1 works as an input polarity guard cum reverse current flow
preventer. Capacitor C1 is a simple buffer for circuit stabilization.
After succesful construction, preferably on a small piece of general
purpose PCB, enclose the whole circuit in a
suitable and attractive pen torch cabinet. If necessary, drill suitable
holes in the cabinet to attatch the dc socket, on/off switch and the
input indicator etc. In prototype,commonly available 4.8 volt/500mah
Ni-Cd battery pack (for cordless telephones) is used.

One very simple but reliable ac mains powered battery charger
circuit for the handy pen torch is also included here. Basically the pen
torch circuit is a constant current charger wired around Transistor T1
(BC636), powered by a 12v/350mA step down transformer and associated
componentsD1, D2 and C1.

AC mains powered battery charger for the pen torch

Unregulated 12 volt dc available from the input power convereter circuit, comprising step down transformer(TRF), rectifier diodes (D1,D2) and filter capacitor (C1), is fed to T1 through a current limiting resistor R1. Grounded base PNP
transistor T1 here works as a constant current generator. With 22 ohm
resistor for R1, the charging current available at the output of the
charger is near 50mA.

Red LED (D3) provides a fixed voltage
reference to the base of T1, with the help of resistor R2. (During
charging process, Diode D1 in the main circuit prevent reverse current
flow from the battery pack when charging input supply is absent.) After
construction of the pen torch circuit, fit the assembled unit inside a
small plastic enclosure for safety and convenience.

Circuit Source: DIY Electronics Projects


Power Buzzer

How often on average do
you have to call members of your family each day to tell them that
dinner is ready, it’s time to leave, and the like? The person you want
is usually in a different room, such as the hobby room or bedroom. A
powerful buzzer in the room, combined with a pushbutton at the bottom of
the stairs or in the kitchen, could be very handy in such situations.
The heart of this circuit is formed by IC1, a TDA2030. This IC has
built-in thermal protection, so it’s not likely to quickly give up the
ghost. R1 and R2 apply a voltage equal to half the supply voltage to the
plus input of the opamp. R3 provides positive feedback. Finally, the
combination of C2, R4 and trimmer P12 determines the oscillation
frequency of the circuit.

Power Buzzer Circuit

Power Buzzer Circuit Diagram

The frequency of the tone can also be adjusted using P1. There is no
volume control, since you always want to get attention when you press
pushbutton S1. Fit the entire circuit where you want to have the
pushbutton. The loudspeaker can then be placed in a strategic location,
such as in the bedroom or wherever is appropriate. Use speaker cable to
connect the loudspeaker. Normal bell wire can cause a significant power
loss if the loudspeaker is relatively far away. The loudspeaker must be
able to handle a continuous power of at least 6 W (with a 20-V supply

The power quickly drops as the supply voltage decreases (P = Urms 2 /
RL). The power supply for this circuit is not particularly critical.
However, it must be able to provide sufficient current. A good nominal
value is around 400 mA at 20 V. At 4 V, it will be approximately 25 mA.
Most likely, you can find a suitable power supply somewhere in your
hobby room. Otherwise, you can certainly find a low-cost power supply
design in our circuits archive that will fill the bill!

Author: G. Baars
Copyright: Elektor Electronics

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Logic PSU With Over-Voltage Protection

A simple 5 Volt regulated PSU featuring overvoltage protection. The 5 volt regulated power supply for TTL
and 74LS series integrated circuits, has to be very precise and
tolerant of voltage transients. These IC’s are easily damaged by short
voltage spikes. A fuse will blow when its current rating is exceeded,
but requires several hundred milliseconds to respond. This circuit will
react in a few microseconds, triggered when the output voltage exceeds
the limit of the zener diode. This circuit uses the crowbar method,
where a thyristor is employed and short circuits the supply, causing the
fuse to blow. This will take place in a few microseconds or less, and
so offers much greater protection than an ordinary fuse.

Circuit diagram:

Logic PSU With Over Voltage Protection Circuit

Logic PSU With Over-Voltage Protection Circuit Diagram

If the output voltage exceed 5.6Volt, then the zener diode will
conduct, switching on the thyristor (all in a few microseconds), the
output voltage is therefore reduced to 0 volts and sensitive logic IC’s
will be saved. The fuse will still take a few hundred milliseconds to
blow but this is not important now because the supply to the circuit is
already at zero volts and no damage can be done. The dc input to the
regulator needs to be a few volts higher than the regulator voltage. In
the case of a 5v regulator, I would recommend a transformer with
secondary voltage of 8-10volts ac. By choosing a different regulator and
zener diode, you can build an over voltag trip at any value.


Transistorised Code Lock With Torch

This electronic lock for
domestic use opens only when you connect the right combination of five
switches. There are twelve switches in total. If you connect a wrong
combination, the lock remains closed. At night, flip switch S3 to ‘on’
position in order to enable the torch. In the daytime, flip it back to
‘off’ position. Fig. 1 shows the circuit of the transistorised code lock
with torch. For easy understanding, the entire circuit can be divided
in three sections: power supply, control and torch.

Circuit diagram:

The original image is sized 768×302px. The power supply section is
built around transformer X1, bridge rectifier comprising diodes D1
through D4 and regulator IC 7812 (IC1). The 230V AC, 50Hz AC mains is
stepped down by transformer X1 to deliver a secondary output of 15V, 250
mA. The transformer output is rectified by the bridge rectifier,
filtered by capacitor C1 and regulated by IC1. Capacitor C2 bypasses the
ripples present in the regulated supply. When mains power is available,
IC1 provides regulated 12V to the circuit and power-on LED1 glows to
indicate that the circuit is enabled.

The control section is built around switches S1 through S12,
transistor T1 and relay RL1. Relay driver transistor T1 is used to
energise/de-energise the relay.The torch section is built around six
white LEDs (LED2 through LED7) and resistors
R3 and R4.Working of the circuit is simple. To open the door, you should
know the connection code. Here the connection code is switches S1, S7,
S2, S11 and S9. This means you need to connect theses witches to each
other by flipping them to ‘on’ position. As the connection completes,
transistor T1 conducts and relay RL1 energises. As a result, the

door lock connected between the pole and normally-open contacts of
relay RL1 opens.If you connect a wrong combination, say, switches S4,
S10, S11 and S6, transistor T1 does not conduct and relay RL1 remains
de-energised. As a result, the door lock remains closed. Assemble the
circuit on a generalpurpose PCB and house it in a small cabinet. Fig. 2 shows the proposed cabinet arrangement for switches and LEDs. Install the cabinet at the front door of your house.

As mentioned in the beginning, switch S3 is used to enable the torch. When it is flipped to ‘on’ position, all the LEDs
(LED2 through LED7) glow. Switches S4, S5, S6, S8, S10 and S12 are used
just to confuse the intruders and play no role in opening the door. You
can also use a 12V battery to power the circuit. In that case, remove
transformer X1, diodes D1 through D4, capacitor C1 and regulator IC2
(7812) and connect the battery inside the cabinet with proper polarity.

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Blown fuse indicator circuit diagram

This blown fuse
indicator will work with a wide range of DC supply voltages from 5V to
50V. It illuminates LED1 when the fuse blows. With the fuse intact, Q1
is held off and there is no bias current available for the base of Q2.
So the LED is off. When the fuse blows, a
small current flows via the base-emitter junctions of Darlington
transistor Q1, through its base resistor R1 and then via the load.
Typically this current will be around 20μA and this turns on Q1 which
provides base current to Q2 which then turns on to illuminate the LED.

Blown Fuse Indicator Circuit

Blown Fuse Indicator Circuit Diagram

The emitter current of Q2 is limited by Q3 which turns when the
current reaches about 10mA, to shunt base current away from Q2. The
three resistor values not given in the circuit are dependent on the
supply voltage and can be calculated from the following simple

  • R1(kΩ) = V(DC)/0.02 = 560kΩ for 12V DC
  • R2(kΩ) = V(DC)/2 = 5.6kΩ for 12V DC
  • R3(Ω) = V(DC)/0.02 = 560Ω for 12V DC

R3 should be included for voltages above about 20V otherwise the
heat dissipation in Q2 will be too great. At lower voltages it can be
omitted. Any general purpose NPN transistors can be used for Q2 and Q3, provided they will handle the DC supply voltage. The PNP Darlington, Q1, could be an MPSA65, available from Dick Smith Electronics (Cat Z-2088).

Author: Keith Gooley – Copyright: Silicon Chip

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On And Off Button

In this simple circuit
we give the chip a little more attention than usual. It is astonishing
what can be built with a 555. We are always infatuated with simple
circuits using this IC, such as the one shown here. The 555 is used here
so that a single push-button can operate a relay. If you press the
button once, the relay is energized. When you press it again the relay
turns off. In addition, it is possible to define the initial state of
the relay when the power supply is switched on. The design is, as
previously mentioned, very simple. Using R1 and R2, the threshold and
trigger inputs are held at half the power supply voltage.

Circuit diagram:

On/Off Button Circuit

On/Off Button Circuit Diagram

When the voltage at the threshold pin becomes greater that 2/3 of
the power supply voltage, the output will go low. The output goes high
when the voltage at the trigger input is less than 1/3 of the power
supply voltage. Because C2, via R3, will eventually have the same level
as the output, the output will toggle whenever the push-button is
pressed. If, for example, the output is low, the level of the trigger
input will also become low and the output will go high! C1 defines the
initial state of the relay when the power is applied. If the free end of
C1 is connected to Vcc, then the output is high after power up; the
output is low when C1 is connected to ground.

Author: Ger Langezaal – Copyright: Elektor Electronics

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Circuit Project: 12KV High Voltage Generator

The hobby circuit below
uses an unusual method to generate about 12,000 volts with about 5uA of
current. Two SCRs form two pulse generator circuits. The two SCRs discharge a 0.047uF a 400v capacitor through a xenon lamp trigger coil at 120 times a second.

The high voltage pulses produced at the secondary of the trigger
coil are rectified using two 6KV damper diodes. The voltage doubler
circuit at the secondary of the trigger coil charges up two high voltage
disc capacitors up to about 12KV. Although this circuit can’t produce a
lot of current be very careful with it. A 12KV spark can jump about
0.75 of an inch so the electronic circuit needs to be carefully wired
with lots of space between components.

Source: DiscoverCircuits


Mains Manager

Very often we forget to
switch off the peripherals like monitor, scanner, and printer while
switching off our PC. The problem is that there are separate power
switches to turn the peripherals off. Normally, the peripherals are
connected to a single of those four-way trailing sockets that are
plugged into a single wall socket. If that socket is accessible, all the
devices could be switched off from there and none of the equipment used
will require any modification. Here is a mains manager circuit that
allows you to turn all the equipment on or off by just operating the
switch on any one of the devices; for example, when you switch off your
PC, the monitor as well as other equipment will get powered down

You may choose the main equipment to control other gadgets. The main
equipment is to be directly plugged into the master socket, while all
other equipment are to be connected via the slave socket. The mains
supply from the wall socket is to be connected to the input of the mains
manager circuit. The unit operates by sensing the current drawn by the
control equipment/load from the master socket. On sensing that the
control equipment is on, it powers up the other (slave) sockets. The
load on the master socket can be anywhere between 20 VA and 500 VA,
while the load on the slave sockets can be 60 VA to 1200 VA. During the
positive half cycle of the mains AC supply, diodes D4, D5, and D6 have a
voltage drop of about 1.8 volts when current is drawn from the master

Diode D7 carries the current during negative half cycles. Capacitor
C3, in series with diode D3, is connected across the diode combination
of D4 through D6, in addition to diode D7 as well as resistor R10. Thus
current pulses during positive half-cycles, charge up the capacitor to
1.8 volts via diode D3. This voltage is sufficient to hold transistor T2
in forward biased condition for about 200 ms even after the controlling
load on the master socket is switched off. When transistor T2 is ‘on’,
transistor T1 gets forward biased and is switched on. This, in turn,
triggers Triac 1, which then powers the slave loads. Capacitor C4 and
resistor R9 form a snubber network to ensure that the triac turns off
cleanly with an inductive load.

Circuit diagram:

Mains Manager Circuit

Mains Manager Circuit Diagram

LED1 indicates that the unit is operating. Capacitor C1 and zener
ZD1 are effectively in series across the mains. The resulting 15V pulses
across ZD1 are rectified by diode D2 and smoothened by capacitor C2 to
provide the necessary DC supply for the circuit around transistors T1
and T2. Resistor R3 is used to limit the switching-on surge current,
while resistor R1 serves as a bleeder for rapidly discharging capacitor
C1 when the unit is unplugged. LED1 glows whenever the unit is plugged
into the mains. Diode D1, in anti-parallel to LED1, carries the current
during the opposite half cycles. Don’t plug anything into the master or
slave sockets without testing the unit.

If possible, plug the unit into the mains via an earth leakage
circuit breaker. The mains LED1 should glow and the slave LED2 should
remain off. Now connect a table lamp to the master socket and switch it
‘on’. The lamp should operate as usual. The slave LED
should turn ‘on’ whenever the lamp plugged into slave socket is
switched on. Both lamps should be at full brightness without any
flicker. If so, the unit is working correctly and can be put into use.

The device connected to the master socket must have its power switch on
the primary side of the internal transformer. Some electronic equipment
have the power switch on the secondary side and hence these devices
continue to draw a small current from the mains even when switched off.
Thus such devices, if connected as the master, will not control the
slave units correctly.

Though this unit removes the power from the equipment being
controlled, it doesn’t provide isolation from the mains. So, before
working inside any equipment connected to this unit, it must be
unplugged from the socket.


Protect a Vehicle-Reverse Camera

The circuit in this
Design Idea uses a simple comparator circuit to make a power-on time
delay for an automotive rear view camera. Auto manufacturers typically
power reverse-view cameras from the reverse-light circuit. In
automatic-transmission vehicles, a short power pulse is applied to the
camera when you shift through reverse as you go from park to drive, or
vice versa. This sudden voltage pulse is bad for the sensitive circuits
in the camera and may reduce its lifetime.

This Design Idea suggests a simple and cheap method for avoiding
this situation.Simple circuit helps to protect a vehicle-reverse camera
figure 1The input to this circuit connects to the positive and negative
terminals of the reverse light (Figure 1). The circuit powers the camera
using a MOSFET. R1 and C1 form a time-delay
element (Reference 1). When the reverse light turns on, it slowly
charges the capacitor through resistor R1. R3 and R4 form a voltage
divider, which you use to set 6V on the inverting pin of the comparator.

At the instant of power application to the circuit, the comparator output is low, and the MOSFET is off. Once the voltage of C1 rises above 6V, the comparator’s output becomes high, and the MOSFET
turns on. The values of R1 and C1 set the time delay to 2.2 sec. You
can calculate this time based on the exponential charging of a capacitor
using the following equations:

You can set a different time delay by changing the value of R1 or
C1. When you shift the gear lever from the reverse position to any other
position, capacitor C1 discharges within 60 msec through D1, R3, and
R4. As you pass through reverse, shifting between park and drive, the
camera does not turn on due to the 2-sec delay.


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Screenshoot of FreePCB

Screenshoot of FreePCB