Unintuitive flasher (LED) - Page 1
Forum section: electronics / beginners

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 30, 2020, 01:29:59 pm
content: I was looking for a simple, super-low power, low parts-count/cost flashing LED circuit and found this (attached).I guess the cap charges and when it reaches a certain voltage, the tranny "breaks down", conducts and discharges the cap quickly. It seems to work down to 9.76V.However, I can't find the emitter-collector breakdown voltage for the 2N2222 - every other combination though!Questions:1. Is this what is happening?2. Does it damage / reduce the life of the tranny?3. Where can I find emitter-collector breakdown voltages?4. Is there a circuit like this that would work on 5V?Cheers.

author: ledtester
author_rank: Super Contributor
timestamp: June 30, 2020, 03:41:05 pm
content: > 4. Is there a circuit like this that would work on 5V?A single UJT can be used to create a simple relaxation oscillator:https://www.electroschematics.com/mock-flasher-led/but whether you can get it to work at 5V is another question.

author: themadhippy
author_rank: Super Contributor
timestamp: June 30, 2020, 04:09:19 pm
content: In the days of chips with everything you'd just chuck in a LM3909,less flexible would behttps://www.rapidonline.com/Catalogue/Product?Id=82-0030

author: TimFox
author_rank: Super Contributor
timestamp: June 30, 2020, 04:35:55 pm
content: Here is a data sheet for the 2N2222A:https://my.centralsemi.com/datasheets/2n2221a.PDFNote that the BVceo (breakdown voltage from collector to emitter with base open-circuit) is listed as minimum 40 V, while BVcbo (collector-base, emitter open) is minimum 75 V.The 2N2221, 2N2221A, 2N2222, and 2N2222A are NPN transistors in metal TO-18 cases. Variations (e.g., PN2222) in plastic cases may be different and you should check the manufacturer's data sheet.The 2N series parameters are registered with JEDEC, or whatever they're calling it now.

author: mikerj
author_rank: Super Contributor
timestamp: June 30, 2020, 04:39:31 pm
content: Quote from: PerranOak on June 30, 2020, 01:29:59 pmYes, a transistor biased in this way suffers from avalanche breakdown.Quote from: PerranOak on June 30, 2020, 01:29:59 pmYes it does. You will find the gain of the transistor reduces after reverse breakdownQuote from: PerranOak on June 30, 2020, 01:29:59 pmTransistors are not designed to be used like this since it damages them, so you won't find specifications relating to this mode of operation.Quote from: PerranOak on June 30, 2020, 01:29:59 pmI doubt there is anything that uses a common bipolar transistor, the exact breakdown voltage varies between transistors but it's always somewhat above 5v IME.

author: magic
author_rank: Super Contributor
timestamp: June 30, 2020, 09:04:31 pm
content: 3. See BE junction reverse breakdown voltage. E to C will be similar, because once charge carriers from E enter B it's a nobrainer to make the final leap to C.2. Maybe. Dunno. Does anyone really know how much? Transistors are cheap, build a blinker and see if it still works after a few years

author: PerranOak
author_rank: Frequent Contributor
timestamp: July 01, 2020, 04:42:39 pm
content: Thank you, cheers all.The gain of the tranny reduces but does it affect the life of it in this application - I don't have years to wait.Maybe a better question, initially, would have been: how can I make a LED flash every, say, 5s for a very long time - a week - without requiring very much power?This is for a microcontroller project that goes to sleep then wakes every hour, takes a measurement then sleeps again. Problem is, you can tell if it's on and running or if the battery has gone (design flaw)! However, it needs to draw very little current or the project will certainly drain the battery (5V). Also, it has to be small and cheap.

author: edavid
author_rank: Super Contributor
timestamp: July 01, 2020, 06:19:21 pm
content: Have you tried having the microcontroller wake up every few seconds to flash the LED?If that draws too much current, a simple oscillator using 4000 CMOS should be fine. I'm not sure if standard inverters (CD4069) or Schmitt triggers (74C14/CD4093) would draw less average current - that's a good experiment for you to do.What is your battery capacity? What is the microcontroller active and sleep current?

author: capt bullshot
author_rank: Super Contributor
timestamp: July 01, 2020, 06:40:33 pm
content: Quote from: edavid on July 01, 2020, 06:19:21 pmI'd second that idea. Flashing an LED by waking up the uC every few seconds should have very minimal effect on the battery if done correctly with a suitable modern uC. It's not uncommon having such a configuration lasting one year or longer using a CR2032 (for the LED flashing part only, can't judge your measurement task without knowing it).

author: Jeff1946
author_rank: Regular Contributor
timestamp: July 01, 2020, 08:32:01 pm
content: Use an LMC555 the cmos version of Dave's favorite chip. It draws less than 100uA at 5 V. It will work down to 1.5V. Two AA or AAA batteries to power should last at least a year.

author: mikerj
author_rank: Super Contributor
timestamp: July 01, 2020, 10:10:51 pm
content: The LM3909 was designed to flash an LED from a 1.5v cell using negligible current, but sadly it hasn't been in production for a long while.This circuitfrom IanB might be of interest.

author: PerranOak
author_rank: Frequent Contributor
timestamp: July 02, 2020, 10:01:33 am
content: Thanks very much all.I don't have figures for the final project but, based on past ones, I wanted a low power option.I had assumed, before researching it, that there would be a simple, analogue solution. I guess I'll just use the uC to do it. Cheers.