Hi I'm just starting to understand electronics but I started the easy way out and bought a kit. It's an optical relay kit. I wanted to use this as an electronic air rifle target. Basically I intend to mount a laser on my air rifle with a switch connected to the trigger and when I "hit" the target, well actually if I'm on target and the laser beam hits the LDR the relay is energised and powers a beeper/buzzer for a second or two. Anyway I received the kit and soldered all the components which as far as I know are all in their proper place and direction. At first impression everything seems to be working as it should with the power LED coming on when 12 volts is supplied and the 2nd LED coming on when a bright light is shown on the LDR. The problem is that as soon as I power the kit the relay becomes instantly energised and remains so. As I understand this should only become energised only when the LDR is hit by bright light. At first I thought that the logic is reversed and the relay de-energises when activated by the LDR but it just remains on. Can anyone please guide me to what might be wrong and what should I check I have included the circuit, thanks.

 

The most likely situation is that you installed D2 backwards, or that it is shorted.
The next most likely situation is that Q3 is either shorted or installed improperly.

 

Thanks for your quick reply, D2 and Q2 are installed the proper way as these kits are almost foolproof as you have the components printed on the circuitboard itself, but I will take them out and check or replace them. What I failed to mention is that I used an unregulated 12v power supply and the output when connected to the board was actually closer to 15v, could this have caused a problem to one of the ICs ? I did try with a 9v battery afterwards without any load on the relay and the same problem was occuring but the damage could have already been done.

 

I would start checking where I said to check.

Your LED2 is lighting up at the proper time, so the circuit is good up to the junction of R11 and 555 pin 3.

The problem lies in the area of R12, Q3 D2, and/or D1.

 

Dismantled and tested R12, Q3, D2, and D1.
All OK

Any ideas what else could be wrong?

 

Another thing that's puzzling me is the fact that both the L2 LED and the relay are activated by 3 on the timer IC, so in actual fact they should both be ON at the same time, I can see why SgtWookie pointed to the components that come in play with the relay. They should also be governed by VR2, please correct me if I'm wrong and yet adjusting the variable resistor to both extremes doesn't do anything. I mean shouldn't it delay L2 on the ON position as it should do on the relay? L2 is only on as long as there's enough light on the LDR, or is L2 independent from the relay regards the timer?

 

VR2 should change the amount of time it takes for the LED2 and relay to drop out after the light on the LDR has dimmed/gone out.

You may have a "solder splash" causing a short to ground somewhere, like on the ground side of the relay (collector of Q3, cathode of D2, anode of D1) or a short to +V on the base of Q3.

How did you test the parts?

 

I think I found out what the problem is. I just noticed that the power ON LED that lights up as soon as the circuit is powered is in fact L2 when it should be L1, that is why the relay is energising as well. On further examination I also noticed that I may have the timer IC soldered the wrong way, does this make sense now. I will take it out tomorrow and test it again. I feel like a real idiot now, because I should have checked as the NE555P IC has a small hole that now I guess is the indicator to how it should be positioned on the board.