Long-time C-MOS timing circuit that can be set from 10 to 100 seconds

Function of the circuit

If you want to use the 555 chip to form a long-time timing circuit, you can use a high resistance value for R to lengthen the CR time constant. However, since the input bias current of the internal comparator is large, it is difficult to charge to the threshold voltage and the comparator cannot be driven. For this reason, this circuit uses a C-MOS timer chip with a very small bias current and selects high-resistance resistors to make the timing time reach 10~100 seconds.

How the Circuit Works

The working process of this circuit is basically the same as that of the 555 timer, but its working current is very small. In order to use differential pulse to trigger, C coupling is adopted. As long as the voltage of pin 2 is lower than VOO/3, the trigger can be triggered.

The timing time is determined by the combined resistance of capacitor C3 and R4+VR1, that is, T=C3(R4+VR1). C-MOS timer chip allows the use of large timing resistors, but if the insulation resistance of C3 is small, it may not reach the threshold voltage of (2/3) VDD, so do not use aluminum electrolytic capacitors, and use low-leakage tantalum capacitors. VR1 can be 1M or higher, but it is difficult to obtain high-resistance variable resistors.