## Tuesday, July 9, 2024

How to create a simple timed flasher using a relay / without a built-in transistor

The operation of this circuit is based on the principle of the transistor connected in reverse bias. The emitter connected to the positive pole of the capacitor a relatively low reverse breakdown voltage. If this breakdown voltage is exceeded, the emitter-collector junction of the transistor becomes conducting and the transistor enters into conduction in the emitter-collector direction. This is why we need to use a relatively large voltage of around 12V. If the voltage is too low, say 5V, for example, the transistor will not reverse conduct and the LED will not light up. This phenomenon is due to the fact that the breakdown voltage of the collector is much higher than that of the emitter.

For example, the data sheet for the 2N3904 transistor tells us an emitter breakdown voltage of 6V while it is 60V for the collector.

The other particularity of this assembly is that the base must not be connected.

The flashing is based on the time constant (rate of charging and discharging of the capacitor) of the RC circuit connected to the emitter of the transistor. This time constant determines the period "t" of the signal and therefore the frequency of the flashing since f = 1/t.

So to increase the frequency, and therefore reduce the time constant, it will be enough to decrease R or C. Conversely, an increase in R or C will reduce the frequency by increasing the time constant.

Obviously, the resistor R can be a potentiometer which will allow easy modification of the frequency.

The shape of the signal being linked to the charging and discharging rate of the capacitor, we can see with the oscilloscope that it is a sort of sawtooth whose ramp is not linear.

The amplitude of the signal is dependent on the supply voltage.

How to create a simple timed flasher using a relay / without a built-in transistor

The operation of this circuit is based on the principle of the transistor connected in reverse bias. The emitter connected to the positive pole of the capacitor a relatively low reverse breakdown voltage. If this breakdown voltage is exceeded, the emitter-collector junction of the transistor becomes conducting and the transistor enters into conduction in the emitter-collector direction. This is why we need to use a relatively large voltage of around 12V. If the voltage is too low, say 5V, for example, the transistor will not reverse conduct and the LED will not light up. This phenomenon is due to the fact that the breakdown voltage of the collector is much higher than that of the emitter.

For example, the data sheet for the 2N3904 transistor tells us an emitter breakdown voltage of 6V while it is 60V for the collector.

The other particularity of this assembly is that the base must not be connected.

The flashing is based on the time constant (rate of charging and discharging of the capacitor) of the RC circuit connected to the emitter of the transistor. This time constant determines the period "t" of the signal and therefore the frequency of the flashing since f = 1/t.

So to increase the frequency, and therefore reduce the time constant, it will be enough to decrease R or C. Conversely, an increase in R or C will reduce the frequency by increasing the time constant.

Obviously, the resistor R can be a potentiometer which will allow easy modification of the frequency.

The shape of the signal being linked to the charging and discharging rate of the capacitor, we can see with the oscilloscope that it is a sort of sawtooth whose ramp is not linear.

The amplitude of the signal is dependent on the supply voltage.