Simplest yet durable light sensor.

[Gforce007]

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.

 

[Nigel Goodwin]

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.

Check for exactly what you want.

 

[Gforce007]

Ok thanx for that info.
but what if there will be a diode across the rellay and one more transistor is added after Q1.

 

[Nigel Goodwin]

Ok thanx for that info.
but what if there will be a diode across the rellay and one more transistor is added after Q1.

It will still be poor - the opamp solution is VASTLY better, and works as it should.

 

[xxxxxxxxxxxxxx]

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))

 

[Gforce007]

Ok how about this one

**broken link removed**

 

[ericgibbs]

Ok how about this one

**broken link removed**

hi,
The link has disappeared.?

Post your image to the forum using the manage attachments button on the Reply window.

EDIT: its just reappeared.!

 

[Gforce007]

hi,
The link has disappeared.?

Post your image to the forum using the manage attachments button on the Reply window.

EDIT: its just reappeared.!

oh, can u see it now....sorry man...

 

[ericgibbs]

hi,
Thats looking better.

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.

 

[Gforce007]

hi,
Thats looking better.

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.

 

[Nigel Goodwin]

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.

A 741 (although very old and low spec), is perfectly fine for this very undemanding application.

It is a good idea to add a little positive feedback as Eric suggested though.

 

[Hero999]

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.

 

[Willbe]

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.

 

[ericgibbs]

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.

hi willbe,
If he adds a hysteresis resistor as I have suggested, that will also give 'snap' action.


BTW: I know you were not serious about satan in your post..

 

[Hero999]

Unless the photo transistor has a base lead, there is no way to add hysteresis to the circuit origionally posted.

It's possible to make a Schmitt trigger with two BJTs but it's better to use a comparator like the LM311.

 

[ericgibbs]

Unless the photo transistor has a base lead, there is no way to add hysteresis to the circuit origionally posted.

It's possible to make a Schmitt trigger with two BJTs but it's better to use a comparator like the LM311.

Really hero,
You must start to read the earlier posts as your replies are out of context.

The one I am referring to is the 741 OPA version.

 

[Willbe]

I know you were not serious about satan in your post..

It turns out computers are the Work Of Bill Gates.

 

[clr2cont]

LDR value

**broken link removed**

In this circuit what would be a good starting value LDR1 to sense darkness?

Thanks

 

[Willbe]

**broken link removed**

In this circuit what would be a good starting value LDR1 to sense darkness?

Thanks

From the circuit it looks like they are expecting 10kΩ as the threshold between light and dark.

 

[oishi]

is there any other way to use the circuit without depending to the LDR?