I've made up a laser security sytem that uses a PNP and NPN transistor.

Everything is going well except for the LDR.

When there is no light the LED will stay off, and once there is light it will turn on. How do I reverse it?

Here is my circuit diagram:

enter image description here

  • 2
    \$\begingroup\$ So which is the photoresistor? \$\endgroup\$ Sep 17, 2015 at 1:00
  • \$\begingroup\$ The PNP transistor looks to wired incorrectly. Can you edit the schematic so that it is correct? \$\endgroup\$ Sep 17, 2015 at 1:56
  • \$\begingroup\$ Sorry im a newbie with this :3 \$\endgroup\$ Sep 17, 2015 at 2:32
  • \$\begingroup\$ How about if you describe how your circuit works, and then we can figure out how to change it. \$\endgroup\$ Sep 17, 2015 at 13:43
  • \$\begingroup\$ @GeorgeHerold The pnp and npn are used as a latch switch.. When there is a laser the led is supposed to stay off.. And once the laser is tripped the led is supposed to go off. And will stay on until you ground the circuit. \$\endgroup\$ Sep 17, 2015 at 13:55

2 Answers 2


enter image description here

Looks like i found a solution for your question,,,

If you used the LDR in the feedback resistor chain of an opamp, then that ought to work as you require...

You'd need to pay close attention fixed resistance(10K) though...probably a bit of experimentation would be in order depending upon LDR behaviour,

You can also exchange LDR place with that 10k resistance,

Solution 2(WITH SINGLE BJT!):


simulate this circuit – Schematic created using CircuitLab

The concept here is simple you connect 2 series resistance (1Pot to experiment) such the voltage drop is sufficient to turn on the transistor form the source at the same time allowing most of the current to flow through LED ,voltage at node 1 should be barely enough to turn on the transistor.

When LDR is on as you might be knowing it offers low resistance and current chooses path of least resistance hence transistor will be OFF so will be the LED,,

You might want to consider to add a base resistance but is optional i think so.

hope this helps

  • \$\begingroup\$ Thanks for the answer sir..ill be sure to try it :-) but im trying to create a circuit that uses npn and pnp transistors.. Is there a way you can help me out with that? \$\endgroup\$ Sep 18, 2015 at 14:45
  • \$\begingroup\$ I first tried with transistors,,, but couldn't,, I will try and let you know if I get an answer \$\endgroup\$
    – Lokanath
    Sep 18, 2015 at 15:21
  • \$\begingroup\$ But will it stay on once triggered? \$\endgroup\$ Sep 19, 2015 at 0:12
  • \$\begingroup\$ What if i use a dpdt switch? \$\endgroup\$ Sep 19, 2015 at 1:08

I am sorry i could'nt simulate it.. Consider pototiometer is the photoresistor and pnp transistor and pull down resistor that keeps the base low at night..led on.. In the day the photoresistor will have low resistance so the base will be high the led off.. Please simulated first to know if and what resistors you wil need with the led .. and if you need resistor in serie with photoresistor..enter image description here

  • \$\begingroup\$ Once triggered will the Led stay on?? \$\endgroup\$ Sep 19, 2015 at 0:14
  • \$\begingroup\$ What if i use a dpdt switch? \$\endgroup\$ Sep 19, 2015 at 1:08
  • \$\begingroup\$ Or a 555 timer? Because im required to only use bjts, 555, relays \$\endgroup\$ Sep 19, 2015 at 11:04
  • \$\begingroup\$ Use a relay as self supply or another pnp to keep npn trigered \$\endgroup\$
    – user86856
    Sep 19, 2015 at 13:10

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