need help with transistor for driver - Electronic Circuits Projects Diagrams Free

goodpickles · 28th January 2004, 02:16 AM

I need help decideing what transistor I should use to drive a relay. I have a photo cell the resistance on the OFF of state is 1K and on the ON state is 490ohms. My relay is 12 volts...on testing it kicks in at 6 volts and out at 2.

I have tried an LM3904 but at 12V that let about 2.3 volts leak threw.

Should i try a fet transistor.

I also have a moterola 8844 would that one work.

Screech · 28th January 2004, 08:55 AM

try an 2n2222a transistor.

ljcox · 30th January 2004, 10:19 PM

The circuit suggested by Screech is crude as it does not have a threshold nor any hystersis. So you need a Schmitt Trigger between the photo resistor and the transistor.

If you use a LM311, you would not need a transistor since it has adequate internal drive capability.

You also said "I have tried a LM3904 but at 12V that let about 2.3 volts leak through"

It is impossible to comment on this without a circuit to look at. If you post one, I'll be happy to help.

Len

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28th January 2004, 02:16 AM	(permalink)
goodpickles	need help with transistor for driver I need help decideing what transistor I should use to drive a relay. I have a photo cell the resistance on the OFF of state is 1K and on the ON state is 490ohms. My relay is 12 volts...on testing it kicks in at 6 volts and out at 2. I have tried an LM3904 but at 12V that let about 2.3 volts leak threw. Should i try a fet transistor. I also have a moterola 8844 would that one work.

30th January 2004, 10:19 PM	(permalink)
ljcox	need help with transistor for driver The circuit suggested by Screech is crude as it does not have a threshold nor any hystersis. So you need a Schmitt Trigger between the photo resistor and the transistor. If you use a LM311, you would not need a transistor since it has adequate internal drive capability. You also said "I have tried a LM3904 but at 12V that let about 2.3 volts leak through" It is impossible to comment on this without a circuit to look at. If you post one, I'll be happy to help. Len