The inverted pulse output from a one shot monostable 555 timer circuit.

Thread Starter


Joined Dec 31, 2019
Hey!! Anyone can help me out on the circuit to get the inverted pulse output from a one shot monostable 555 timer circuit. The circuit seems to be correct but still I am not getting the desired output of 12 volts which drops to gnd for a few seconds and then again raised to 12 volts instead a 7 volt raise is received. I need to drive a 12 volts latch relay with the output. Will require 2 such circuits for dual coil latch set and reset coils. Help is really appreciated, thanks!!


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Audioguru again

Joined Oct 21, 2019
With your 12V power supply, the output pulse of the 555 is about +10.6V and the Vbe of the transistor is about 0.7V. Then the 1k base resistor gets a pulse of 9.9V resulting in a base current pulse of 9.9mA.

Most transistors conduct well when the base current is 1/10th the collector current so with your 9.9mA base current, the transistor will turn on well when the collector current is 99mA or less. The relay coil will get about 11.8V and have a resistance of 11.8V/99mA= 119 ohms or more.

Since your transistor is not turning on well then maybe you have its emitter and collector wires swapped?
Or the relay coil resistance is much less than 119 ohms then the transistor needs a lower base resistance value?

See, we are guessing because you did not say the important details of the resistance of the relay's coil and the transistor's part number.