a timing circuit

[joseph barbuzza]

Once again thank you

 

[joseph barbuzza]

KrisBlueNZ ok so i try the circuit when i got home from work. And it's not working. I have the same results except for from the negative bat to the positive side of the photoresistor is 0. I tried reversing and replacing yet same result. Am i missing something

 

[joseph barbuzza]

Also i removed the lead to the gate and the circuit is working fine as soon as i touch the lead to the gate dogs the voltage to 0

 

[joseph barbuzza]

Reviewing all that you have shown me i stayed back from the beginning. Figured out that the mosfet was damaged. Either by static discharge or a short. Found a good mosfet and the circuit world again. Thank you for all the info and explanations

 

[KrisBlueNZ]

So let me get this straight. If you disconnect the 100Ω resistor between C and D (in post #28) to isolate the phototransistor from the MOSFET's gate, you get voltage at C, which is high when the phototransistor is dark, and low when the phototransistor is lit by the LED?

And when you connect the resistor back to the gate, that voltage is always zero?

If so, it sounds like your MOSFET is damaged. MOSFETs can be damaged quite easily by electrostatic discharge from handling. When you buy them, they should come in anti-static tubing or foam.

You can protect your MOSFET from being damaged by connecting a zener diode permanently between the gate (left pin) and the source (right pin). Do this as soon as you get it home, before you remove the foam. Like this:

 

[joseph barbuzza]

You are right. Replaced the mosfet and the circuit is up and running. I thank you for the info on caring for the mosfet. I had thought I'd i put a ground lead from the mosfet casing to the negative rail it would suffice. Your way sounds better.