Need Help, Light Activated LED Latch, ("You've Got Mail" indicator) - Page 1
Forum section: electronics / beginners

author: Altair8800
author_rank: Regular Contributor
timestamp: August 12, 2024, 06:18:48 am
content: I'm trying to find a circuit that when it detects light will continuously light up an LED, will be low DC voltage (3-6v, preferably 5v), and has a reset.What I really want is a "You've Got Mail" indicator. So, when the mail person opens the mailbox the ambient sunlight will trigger the LED inside our house to go on and stay on. After, when I pick up the mail, I can reset the LED to off (for next time).I kind of imagine having and LDR light sensor (or other) and 2 or more jellybean transistors to do the latching.Is this possible and better yet how exactly?NOTE: I've actually tried numerous times (and days) and went down numerous rabbit holes trying to find this exact circuit. I can find circuits that 2/3 does, but nothing that exactly does what I want. I'm trying to avoid latch circuits that need the LED pointed close and directly at the LDR, or latch circuits that need an electrical/mechanical relay switch.

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 12, 2024, 07:13:56 am
content: You need to look for the standard double npn flip flop circuit...Or if you want to do it with less current consumption then I would take an "inverting Schmidt trigger" logic IC. Just connect two of their gates input to output with resistors that can be pulled down by your control lines. That way you have a perfectly stable and low powered latch.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 12, 2024, 09:41:59 am
content: For checking that doors were opened a small magnet + reed switch are used (there are ready to mount at door versions offered). Using it you get more unambiguous signal than from light detecting (what if the day is cloudy or it is dark longer in winter).What you need to latch that signal is called SR flip-flop (S-Set, R-Reset).See:https://en.wikipedia.org/wiki/Flip-flop_(electronics)

author: SanctePieDecime
author_rank: Contributor
timestamp: August 12, 2024, 01:06:27 pm
content: I am actually working on a project that is very similar (although a lot more detailed):Here is a schematic of the power latch circuit I am implementing:There are plenty of videos out there that explain how a circuit like this works. Here is one that I found explained the concepts well:I am using a normally closed reed switch as the input here, but if you wanted to go with a light detection circuit, you could easily use a comparator to convert the light level into a digital signal and use that. I only have the N-channel MOSFET (Q2) there because I wanted to be able to read the status of the reed switch in another part of the circuit. So, there are ways this circuit can be simplified for your applications.I would recommend using a reed switch if you can because then you can reduce power consumption caused by having a comparator running constantly.There are probably better ways of doing things than the way I am doing them, but this is just one of many legitimate approaches.

author: Smile2016
author_rank: Regular Contributor
timestamp: August 12, 2024, 03:12:39 pm
content: something like this ? you can reset by a push button by shorting base and emitter Q2 .

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2024, 03:18:22 pm
content: The normal transistor latch will work. R5 ensures the LED switches off properly. SW1 is the reset.Here's an example. It's not designed for low power consumption, which can be reduced if necessary. Changing Q1 to a MOSFET such as the 2N7000 will enable much higher resistor values to be used.

author: Altair8800
author_rank: Regular Contributor
timestamp: August 12, 2024, 06:07:21 pm
content: First, thanks for all the replies.I'll test out a few to see which one works (and best for our house)...Give me a few days and I'll try and report back results/findings...Cheers...

author: Smile2016
author_rank: Regular Contributor
timestamp: August 12, 2024, 08:00:17 pm
content: "enhanced revision here.. keep in mind there is no delay between "reset" [temporary short R4 with push button] and new trigger. you have to reset in dark mode ...cheers

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2024, 09:16:21 pm
content: Quote from: Smile2016 on August 12, 2024, 08:00:17 pmDo you realise you've designed a thyristor there? Yes, it'll work.https://en.wikipedia.org/wiki/Thyristor?useskin=vector#Designhttps://www.mouser.com/datasheet/2/302/BT169B-350538.pdf

author: Smile2016
author_rank: Regular Contributor
timestamp: August 13, 2024, 03:25:58 pm
content: Quote from: Zero999 on August 12, 2024, 09:16:21 pmYes, that’s the idea. As long as the gate trigger current is sufficiently low, you can use a thyristor or triac. In most electronics labs, you’ll typically find a small NPN+PNP laying around. Keep the 1k resistor to limit current , replace group it with a thyristor, and reset by shorting across the anode and cathode. keep the LED and button out of mailbox so you can have dark on LDR while resetting

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2024, 04:56:54 pm
content: Quote from: Altair8800 on August 12, 2024, 06:18:48 amWhat happens if you receive mail when it's dark?Perhaps a switch would be a more reliable solution?

author: Smile2016
author_rank: Regular Contributor
timestamp: August 13, 2024, 06:21:58 pm
content: Quote from: Zero999 on August 13, 2024, 04:56:54 pmA tilt sensor (switch) on the door, assuming the door tilts, or a reed switch with a magnet. idea for schematic for normal close switches bellow..S1 is normal closed, when opens , LED latches ON. reset by temporarily shorting base Q2 to GND...