Hey all,
My wife has an idea and wanted a circuit designed but I’m just a modest beginner and sure I can build it but need help with design. She wants an LED to light up when struck with the beam from a laser like those of pointer laser pens. The LED needs to light when an LDR is struck but then stay on till power is removed from the circuit. The power available would be a 2032 coin cell/3vdc. (Space is a premium for the circuit) In what I’ve seen the latching part so the LED stays on after the light/laser is removed is the contentious part. Any help would be greatly appreciated. There is room for a few other associated components.
THANK YOU

 

I'm working on a similar project so I modified my idea for you. I tested it on the breadboard and standby current is very low. Instead of an LDR, I used a red LED because I found white light in the room caused too much interference. I made the circuit in 4 steps:

1) LEDs make light but they can also produce a current if light shines on it. The amount of current produced is super small so I used 2 PNP transistors (Q1 and Q2) to amplify the current thousands of times. I also used PNP transistors because I want the Set signal to be near 0V when no light hits D1.

2) U1 and U2 form an SR latch. I made mine with NOR gates to be compatible with PNP transistors but this is preference. When light hits D1, Set will be driven high and U2 will turn off. If Reset is driven high with a switch, U2 will turn on.

3) SR latches are sensitive to noise so the output of U2 is buffered by U3 which sources Q3.

4) R1, C1 and R2 add feedback to the circuit so when power is reconnected, Q3 is off by default.

Let me know if it works for you. I'll be doing more tests.

 

Cool project- would it not be cooler if the sensor and the indicator lamp were the same part?
This could be achieved via the photoelectric current produced from the LED when a laser hits it.

 

I think coin -cell battery will not be enough to power up the setup. Laser diodes usually need 5V to operate.

 

I'm working on a similar project so I modified my idea for you. I tested it on the breadboard and standby current is very low. Instead of an LDR, I used a red LED because I found white light in the room caused too much interference. I made the circuit in 4 steps:

1) LEDs make light but they can also produce a current if light shines on it. The amount of current produced is super small so I used 2 PNP transistors (Q1 and Q2) to amplify the current thousands of times. I also used PNP transistors because I want the Set signal to be near 0V when no light hits D1.

2) U1 and U2 form an SR latch. I made mine with NOR gates to be compatible with PNP transistors but this is preference. When light hits D1, Set will be driven high and U2 will turn on. If Reset is driven high with a switch, U2 will turn off.

3) SR latches are sensitive to noise so the output of U2 is buffered by U3 which sources Q3.

4) R1, C1 and R2 add feedback to the circuit so when power is reconnected, Q3 is off by default.

Let me know if it works for you. I'll be doing more tests.

View attachment 307245

Thank You for this INCREDIBLE effort but there is no room for an IC and there will be no PCB. Parts just soldered together in the air if you will. Neatly but nothing formal. Simple and smaller the better. All stuffed into the belly of a small figure(long story…

 

Thank You for this INCREDIBLE effort but there is no room for an IC and there will be no PCB. Parts just soldered together in the air if you will. Neatly but nothing formal. Simple and smaller the better. All stuffed into the belly of a small figure(long story…

What dimensions did you have in mind? Here is a photo of the components. There are 4 NOR gates on the IC. I used a DIP IC and through hole components but there are smaller versions of each available.

You can swap a PNP transistor and the red LED for an LDR and resistor for the same effect. R3 and Q3 can be removed as long as the LED current is < 20mA.

 

An LDR, thyristor and resistor should do it.

 

What dimensions did you have in mind? Here is a photo of the components. There are 4 NOR gates on the IC. I used a DIP IC and through hole components but there are smaller versions of each available.

You can swap a PNP transistor and the red LED for an LDR and resistor for the same effect. R3 and Q3 can be removed as long as the LED current is < 20mA.

View attachment 307284

I’ll breadboard it. Do you have a regular schematic with IC PIN numbers?

 

I’ll breadboard it. Do you have a regular schematic with IC PIN numbers?

The pin numbers for logic gates are usually not shown. Just match the symbols to the pin numbers in the datasheet. I used cd74hc02 NOR gates but any CMOS gate should work.

 

Here's a version using a SCR.
Adjust R4 to set sensitivity.
Standby current appx 80ua

 

Here's a version using a SCR.
Adjust R4 to set sensitivity.
Standby current appx 80ua
View attachment 307327

Should I try making a SCR with two BJTs?

 

Should I try making a SCR with two BJTs?

One could like this. Works the same.

 

One could like this. Works the same.
View attachment 307356

Thank You so much. I will try it out today after work. Appreciate it! This site is always so helpful and the community giving….

 

One could like this. Works the same.
View attachment 307356

I breadboarded this up and the LED comes on and stays on. LDR has no apparent effect. I substituted a 10K POT for the 2.2K resistor for adjustment to no effect?

 

I suspect you connected the transistors incorrectly. It worked fine for me.

 

I’ll check again but I have checked and rechecked. Admittedly not an engineer but breadboarded hundreds of circuits
Thank you and I will redo it from scratch

 

Can you post a HD photo of the breadboard?

 

Can you post a HD photo of the breadboard?

 

 

View attachment 307444