741 problem in light sensitive switch

Discussion in 'The Projects Forum' started by RodneyB, Feb 20, 2015.

1. RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
I have attached a diagram of a light sensitive switch that I built before but now I have a problem and I cant find out why.

Between R1 and R2 I have 5,92 Volts

When the LDR is under a light (LED) the voltage is 5.8 Volts

However on the output (PIN6) despite exposing it to light or covering the LDR I have an output of 12 Volts.

I am lost cause I have made it before only thing that's new is the supplier of the LDR.

File size:
49.1 KB
Views:
53
2. Dodgydave AAC Fanatic!

Jun 22, 2012
5,155
772
what happens to the output if you short out the ldr?

Apr 5, 2008
15,805
2,389
Hello,

What is the resistance of the LDR in light and dark situation.
You can adjust the switching point by changing the value of the 39K at the - input of the opamp.

Bertus

RodneyB likes this.
4. MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Since the circuit has hysteresis (R4), R5 and C1 are not needed and should be eliminated.

R6 (after deleting R5) is way too high to reliably pull in the relay. R6 should be < ~(10V/10mA) = 1K.

The max allowed current for the BC547 is only 100mA, which may be exceeded depending on the coil resistance of the relay.

Apr 5, 2008
15,805
2,389
Hello Mike,

Good catch, perhaps he wants a delay, that he tried to create with R5 and C1.
If the delay is wanted, the signal after R5 and C1 should be buffered or tested for a level.

Bertus

6. RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
I will check and post result

7. MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Since the delay would be ~0.2*(30K//100K)*470e-6 = 2ms, I don't think it is needed or does anything useful.

Apr 28, 2012
652
14

9. RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
Hi I used R4 and C1 to create a 15 second delay but its quite erratic.The delay is for cloud delay. Without this my light flicked on and off. Should I still change the resistor to 1K

10. MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
If the circuit is flickering, then you have it biased too close to the trip point. I would add more hysteresis by reducing the resistance of R4, and eliminate the delay. You were not getting anything close to a 15second delay with R5 and C1.

R6 has to be selected based on getting enough base current to the NPN to saturate it. 1K will saturate it...

Apr 28, 2012
652
14
Thank you

12. RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
Hi

In the Light the resistance is 1.8K
In the dark the resistance is 1.5 Meg

I thought in the light the resistance was 0R and in the dark it was very High at least over 1.5 Meg?

Apr 5, 2008
15,805
2,389
Hello,

You could replace R3 with a 10K resistor with a 100K potentiometer as rheostat in series (the middle pin to one of the outer pins connected).
That way you ca adjust the light sensitivity.
Also you can change R4 to 100K with a 1M potentiometer in series for the switch points.

Bertus

14. MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Here is a better circuit:

I believe that the resistance of the LDR near where you want things to happen is about 5 to 10KΩ, so I reconfigured the bridge so that the trip point is closer to the full sunlight resistance. That might in-and-of-itself cure the cloudy day problem. I also reduced the current through the two arms of the bridge, to reduce the standby current.

I also greatly increased the amount of the hysteresis by increasing the positive feedback around the opamp.

Finally, I took a look at the output circuit, and because the '741 doesn't pull all the way to 0V when low, with the 1K base resistor, I had to add the LED in series. The LED lights up when the relay is pulled in, but more importantly, its forward voltage drop (~2.2V for a red LED) subtracts from V(out) of the opamp to guarantee that the NPN turns off. That is further improved by the 10K from base to gnd.

Last edited: Feb 20, 2015
RodneyB likes this.
15. djsfantasi AAC Fanatic!

Apr 11, 2010
2,907
879
Shouldn't there be a snubber diode across the relay coil?

16. RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
tHAN
Thank you so much I will build it tomorrow,

17. MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
There is in mine

RodneyB likes this.
18. RodneyB Thread Starter Active Member

Apr 28, 2012
652
14
Going to build the correct one tomorrow thanks to Mike

19. AnalogKid Distinguished Member

Aug 1, 2013
4,699
1,298
"...only thing that's new is the supplier of the LDR."

That's probably the problem. If you still have access to the ones you've built before, measure the LDR and compare it to the new ones under similar conditions.

ak

RodneyB likes this.
20. djsfantasi AAC Fanatic!

Apr 11, 2010
2,907
879
Oops, there it is! Missed it earlier.