I am trying to build some device that will emulate the operation of some module in my car. The input is the 12V logic signal from the trasmission control module. Basically, the device should take this signal and invert it, with a 2.5 sec delay. The delay will be created with the help of the astable timer based on LM555 and inverted with the NPN transistor: inverted and delay circuit Since I am somewhat afraid to load the transmission control module of the car, I wanted to use a high side switch between the transmission module and the 555 timer: high side switch.

The input signal is applied to this switch, and the switch activates the 555 circuit. The circuit works but its activation threshold is rather low - the high side switch is activated even with the 2V of input voltage. How can I increase the activation threshold (or reduce the sensitivity if the circuit)? It seems that the input impedance of the LM555 is high so maybe the high side switch is not necessary and an input signal from the transmission module could be applied directly to the timer circuit? Thanks.

  • \$\begingroup\$ If drawing 1-2mA from signal line is not a problem then you can put a zener in series. For example, place a 9V1 zener, reduce R1 and you'll have an input threshold of ~10V. \$\endgroup\$ – Rohat Kılıç Jun 22 '20 at 22:31
  • \$\begingroup\$ Place a resistor from Q1 base to ground. eg 10k will (should) about triple the input threshold. \$\endgroup\$ – Russell McMahon Jun 24 '20 at 10:15

Add a resistor between the Q1's Base and Gnd, name it Rb. Assume you want the circuit to be triggered at input voltage of 6v, so find the value of Rb based on: (Rb / (Rb + R1))*6v = 0.6v, (where 0.6v is Vbe of BJT transistor)


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