I don't know if a transistor biased with a resistor and a photocell or LDR will work fine for a light sensor or not.
Will such circuit be robust and will it work for years and years with out any repair work.....
here is an example.
**broken link removed**
so if it needs some modification (apart from the diode in parallel with the relay which it an't have) then plz tell me...
thank you.
It should work for years, but it will never work very well - and that particular example is REALLY bad, and probably won't work at all as there's no diode across the relay.
Use an opamp as a comparator instead, with a little positive feedback.
The diode acts to dump current backflow to ground. That is, when a relay is closed/opened, current spikes (due to inductance) often flow back through. This will damage your transistors (or any other circuitry, for that matter).
The diode is inserted so that is is reversed biased, that is, it (obviously) wont short out the current during normal operation, but reverse current will be shorted.
Hope this explains why you'll need a diode.
(by the way, the diode should be just any suitable rectifier diode (IN4004 or similar))
I would use a better OPA than the 741, say a LM358 or TC358.
Also add a little positive feedback on the OPA in order to stop the relay chattering on/off at dark/light switch over.
Thats a 1M0 for starters between the output of the OPA to the +NI input of the OPA.
I would use a better OPA than the 741, say a LM358 or TC358.
Also add a little positive feedback on the OPA in order to stop the relay chattering on/off at dark/light switch over.
Thats a 1M0 for starters between the output of the OPA to the +NI input of the OPA.
Oh thanx "ericgibbs", but i guess i have to use the 741. since its easily available in my area.
Ok thanx for the +FB resistor i'll try to add it if the relay will misbehave.....
thanx alote.
Oh thanx "ericgibbs", but i guess i have to use the 741. since its easily available in my area.
Ok thanx for the +FB resistor i'll try to add it if the relay will misbehave.....
thanx alote.
Using a comparator like the LM311 which can drive 50mA is a much more compact solution as it will drive small relays directly without using a driver transistor.
There is a two transistor schmitt circuit where both emitters share a resistor and this gives the triggering action. I don't believe this circuit would give that triggering action, even with a small resistor in the ground lead.
After you add your coil suppressor, with some rewiring I think you can make a schmitt out of the original circuit to get your hysteresis; you might need to add one transistor, though.
There is a two transistor schmitt circuit where both emitters share a resistor and this gives the triggering action. I don't believe this circuit would give that triggering action, even with a small resistor in the ground lead.
After you add your coil suppressor, with some rewiring I think you can make a schmitt out of the original circuit to get your hysteresis; you might need to add one transistor, though.