Transistor as switch -problem


Joined Nov 18, 2012
If I understand correctly you want to flash the LED for 30 seconds at a 2 hertz rate. If this is correct U1 is acting as a monostable timer and the circuit and components values look ok (but more adjustment range on R3 may be needed). U2 looks like it is configured correctly an astable timer with the reset tied to the output of U1, but the component values for U2 might need to be adjusted as follows R5 - 14K and R4 - 140K this will be close to a 50% duty cycle 2Hz flash rate (0.7 / (f*C)) with the 10uF capacitor

f = frequency and C is capacitance in farads.

Checking a spec sheet for the 555 the reset line to U2 needs a pull up as the output of the 555 is an open collector output. That also means the LED anode should be tied to VCC and the cathode to Q of U1. With LED in that arrangement the pull up may not be needed

This site had a lot of good info on the 555 and 556
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