a timing circuit

[joseph barbuzza]

I have a mosefet/photoresistor/LED circuit. It is to replace a basic relay circuit. Ive gotten a circuit design but it doesn't work and can't understand why.

 

[Bluejets]

By the look of that you need more gate voltage, hard to say without knowing component details.
Drawing should follow standard drawing practice, i.e. positive rail at the top, negative rail at the bottom, everything else in between.
Makes for easier reading and troubleshooting.

 

[KrisBlueNZ]

Hi Joseph and welcome to Electronics Point

That circuit is really badly drawn but it should do something.

Let's start by testing the LED/phototransistor section. Set your multimeter to the 20V DC range and connect the red probe to the MOSFET's gate and the black probe to the source.

With the phototransistor covered up, you should measure about 9V DC on the gate. With the phototransistor exposed to the light from the LED, that voltage should drop to less than 1V.

Let us know the results of those tests and we'll go from there.

 

[joseph barbuzza]

Bluejets, i agree but that's what i got to work on. Krisbluenz i get 8.4 volts irregardless. All parts are new. Made the circuit twice. Same results. 24v constant flow and 8.4 volts constant and i have no idea why

 

[KrisBlueNZ]

1. Check that the 330Ω resistor (in series with the LED) is the correct value. It should be orange-orange-brown, or orange-orange-black-black.
2. Check that there is at least 6V across the 330Ω resistor. If there isn't, you may have connected the LED backwards. Try connecting it the other way and see whether the voltage appears across the 330Ω resistor now. If the voltage appears, the LED is now connected the right way, but the LED may have been damaged so you should replace it.
   
3. Make sure that the LED is pointing directly at the phototransistor. Tape them together temporarily if necessary.
4. Check that the 3.3 kΩ resistor from the phototransistor to the +9V rail is the correct value. It should be orange-orange-red, or orange-orange-black-brown.
   
5. The phototransistor may be connected backwards. Try reversing the wires to it and see if that changes the gate-source voltage and turns the MOSFET OFF (when the LED is pointed at the phototransistor).

 

[joseph barbuzza]

Tried but all i get is a .10 v drop when putting the led in front of the phototransitor. It seems like the mosfet is constantly on irregardless

 

[KrisBlueNZ]

1. What voltage do you measure across the 330Ω resistor?

2. What voltage do you measure across the 3.3 kΩ resistor with the phototransistor covered up?

3. What voltage do you measure across the 3.3 kΩ resistor with the LED pointing directly at the phototransistor?

 

[joseph barbuzza]

What voltage do you measure across the 330O resistor? = 8.54V

What voltage do you measure across the 3.3 kO resistor with the phototransistor covered up? = 7.27V

What voltage do you measure across the 3.3 kO resistor with the LED pointing directly at the phototransistor?= 7.29

 

[KrisBlueNZ]

What voltage do you measure across the 330O resistor? = 8.54V

OK, what voltage do you measure across the LED?

What voltage do you measure across the 3.3 kO resistor with the phototransistor covered up? = 7.27V

You've made a mistake. In post #4 you said there was 8.4V from the gate to the source of the MOSFET, and here you say there is 7.27V across the 3.3 kΩ resistor. Those can't both be true unless you've wired the circuit wrong. Double check each component and connection against the diagram.

 

[joseph barbuzza]

Sir, first of all thank you for being kind enough to help. Second, i am helping my father on this project. If you don't mind tomorrow i will send you a picture of the actual circuit soldered and completed. Also with the results you have asked for. I would appreciate any help you could offer. Thank you

 

[KrisBlueNZ]

OK Joseph

 

[joseph barbuzza]

OK Joseph

Hello krisbluenz, here are my results. All results off a 9v battery with 8.81v
Voltage across 330 ohm=8.37
" 3.3k=7.52 w/o LED
" 3.3k=8.20 w/LED
gate-source voltage=7.39 w/o LED
" =8.10 w/LED

 

[KrisBlueNZ]

You've definitely wired something wrong, and/or used the wrong component values. Can you upload enough photos of your circuit so every component and connection is clearly visible, and we'll go from there.

 

[joseph barbuzza]

Let me know if it looks good

 

[joseph barbuzza]

All components are correct. As stated and checked before soldering anything. I got the transitor and soldered the shotkey diode and the capacitor to the source and the drain. Drain is my - output. My 100k ohm resistor is soldered to source and gate on the transistor. Source goes to 24v battery -. And to - LED, - phototransistor. Gate to 100 ohms resistor to + phototransistor to3.3k to +9v and 330ohm to LED

 

[KrisBlueNZ]

That's a start but I said in post #13:

Can you upload enough photos of your circuit so every component and connection is clearly visible, and we'll go from there.

I can't see the LEDs, and I can't read the markings on the transistor. Take as many photos as you need to ensure everything is clearly visible and readable.

 

[joseph barbuzza]

I will do that as soon as i get back from work

 

[joseph barbuzza]

Hello KrisBlueNZ, so tgis is what i have as far as pictures

 

[joseph barbuzza]

Pict2

 

[joseph barbuzza]

Pic3