The push on push off transistor switch

You’re probably familiar with single button on/off switches. Most of the ones you see now on devices are IC logic switches but there was a time when these types of switches used transistors only. I can remember when the first stereos of the 70s had “soft touch” controls. One button turned a device on or off without actually mechanically switching the circuit.
This project demonstrates a simple version of this type of circuit. It uses three transistors, two NPN and one PNP. There’s also a capacitor which gets charged and discharged, and this is what makes the button capable of performing two functions with the same contacts. The output of this circuit is negative.
Here’s a description of the circuit as it appears on the website:

When the circuit is turned on, capacitor C1 charges via the two 470k resistors. When the switch is pressed, the voltage on C1 is passed to Q3 to turn it on. This turns on Q1 and the voltage developed across R7 will keep Q1 turned on when the button is released.
Q2 is also turned on during this time and it discharges the capacitor. When the switch is pressed again, the capacitor is in a discharged state and this zero voltage will be passed to Q3 turn it off. This turns off Q1 and Q2 and the capacitor begins to charge again to repeat the cycle.

This is a useful circuit that can be put to use on many devices. Try building one and see what you can switch on and off with it.

Keep on hackin!

Parts List:
2 – BC547 NPN Transistors
1 – BC557 PNP Transistor
3 – 470K resistors
2 – 10K resistors
1 – 1K resistor
1 – 100K resistor
1 – 1M resistor
1 – 1uF polarized capacitor
1 – Momentary contact switch
1 – 9 volt battery and holder

The Schematic:

4 thoughts on “The push on push off transistor switch

  1. I had the same problem, my fix for this was adding an extra resistor a 1.2k type.
    On the OFF mode, you will have 0.03 volt on output.

    The circuit looks like that:

  2. I added the resistor that RegiKacat suggested but when I hit the momentary switch the LED just blinks but does not switch off. If I hold the button down, making the connection the LED is off and then turns back on when I release the switch. What am I doing wrong?

Comments are closed.