Simple DC Bulb Flasher Circuit

This simple DC bulb flasher circuit has a number of uses in many fields. It is common knowledge that flashing lights are far more noticeable than those that simply remain on. For example, advertising signs which come on and off are far easier seen than those that are simply lit up police cars and ambulances also use flashing lights.

A DC lamp flasher is a device which is designed and used for switching a connected lamp alternately ON/OFF at a certain given rate per second through a DC supply source.

Meaning, a lamp or a light emitting device is switched ON/OFF rapidly at some given rate as preferred by the user, resulting in a blinking effect of the light or flashing effect on the light.

Such lamp flashers are used for warning devices, in police and ambulance vehicles, in airplanes, flashlights or any desired place where an emergency kind of situation needs to be indicated.

Alternatively a lamp flasher is also popularly used for decoration and in vehicles for indicating a turn signal.

In many cases, especially in motor cars, lights are used as warning devices, and it is not a difficult matter to convert these to flashing types rather than simply switch on.

Flashing Light is More Attractive

This makes them very much more apparent and while a straightforward light could be missed, a flashing light can not be. The circuit is shown here for 9V but all that is necessary to convert it to the 12V used in cars is to use a 12V bulb, in fact the existing warning bulb can be used.

The working tolerances of the components have been deliberately chosen to accommodate this higher working voltage. For use on 6V the circuit will also work and the specified bulb will suffice though it will not glow quite as brightly as it would do if it were connected directly to the supply.

However this is made up for as the light is flashing and is therefore much more apparent. The cost of the circuit is very small.

Can be used as a Car Beacon

When used in a car as a beacon light, no battery is needed, nor any switch and the bulb is already in place. This simple bulb flasher circuit is in fact a common multivibrator but unlike most designs the two transistors are not matched and there is an imbalance between the two sides.

This is deliberate since only one of the transistors needs to carry high current, there seems little point in making both power types.

The use of a true power transistor may at first seem wasteful but frequently such a circuit is working under very unfavorable conditions.

In a car with the heater on, or on a very hot day, the temperature inside a car can reach high levels, quite outside the range expected of most circuits. If the circuit is to be used with a battery under normal conditions the AD162 may be replaced by another 2N3702 with no circuit changes being necessary.

NPN transistors may also be used in the circuit but in this case the battery polarity will have to be reversed and the electrolytic (C1 and C2) will have to be wired in the other way around.

How the Circuit Works

Although many people are familiar with the multivibrator, not many are quite sure how it operates. when the supply voltage is applied, one transistor is bound to draw slightly more current than the other, this will be so even if great care is taken to ensure that the two sections are similar.

Let‘s SBY that Tr2 draws more than Trl (though ofcourse the same argument would apply if it was Tr2 that drew the higher current).

This would cause a fall in the voltage at the collector of TrZ. This means that C2 would start to charge and while it does so the potential at the base of Trl would not be enough to maintain conduction.

However, as C2 continues to charge a certain point would be reached when Trl begins to conduct. This in turn causes a voltage drop at the base of Tr2, turning this off and so the cycle continues. When T1-2 is fully on, almost the complete supply voltage will appear across the bulb causing it to light up.

Since it would be a waste of current to reduce the collector load of Tr1 to the low level of Tr2, balance is maintained in the circuit by having different values for Cl and C2.

The flashing rate for the components shown is about one per second, but this can be altered by changing the value of R1. A lower value resistor here will increase the ashing rate, a higher value will slow the rate.

Parts List for the simple DC lamp flasher circuit

• R1 = 82K ohms
• R2 = 1K ohms
• R3 = 22K ohms
• C1 = 50 uF 25V
• C2 = 2 uF 25V
• Tr1 = 2N3702
• Tr2 = AD162 or similar - See Text
• LP1 = 6V 40mA bulb - See Text
• SW1 = On-off switch
• B1 = PPS 9V battery

Another Simple DC Flasher design

In this second example we learn how to make a simple miniature lamp flasher circuit using some inexpensive transistors, resistors and capacitors. Let's learn the details below:

The discussed miniature lamp flasher circuit presented here can be used in various ways. For instance, it may be built into an inexpensive decorative lantern at Diwali time, where it will catch the attention of passersby from as far as 15 to 20 metres.

The circuit can be used to light up one or two bulbs (for two lanterns). When used for a single bulb, the light of course would be much brighter.

The flashing rate of the bulb can be varied through VR1. The entire unit can be housed in a box of 60 x 40 x 15 mm size. It will cost ·approximately Rs 25 only to build. If one desires to operate the unit with a battery eliminator, and C4 can be omitted. But it is advisable to keep Dl in circuit as a protection against wrong polarity connection.