A simple traffic warning light drives me nuts!

[compet17]

I stumbled across a defective traffic warning light (see pic1) and I repaired it... but the function of the circuit is very obscure. Does someone see how that works? It looks very simple but it's far from being obvious.

For example there is an LED connected to the base of BC557 which is probably used as an zener diode. That LED was defective and when I replaced it with a normal red LED of the same size, the light blinked again.

There are 2 switches: ON/OFF and BLINK/STEADY light.
The 2 LEDs connected to BC337 are the signal LEDs that are visible.
Blinking frequency is about 2Hz with a long pause and a short flash. The flashes are not brighter than the steady light.

My question is, how is that capacitor charging and dischaging? How does the BC557 turn on and off? And why did they use a LED in the circuit?

I'm a professional electronics guy and this curcuit looks completely wrong but it works and I can't say why.... maybe you can?

Chris

[strawberry]

invisible LED sensor for day time battery saving
RC to keep time for some basic oscillator

[harerod]

You could feed your circuit into the LTspice - a great free tool to check analogue concepts. The results from LTspice could serve as a basis for further discussion, if required.

[magic]

Meh, it's a simple thing.

Ignore the stupid capacitor, it's an open circuit anyway. The diode and PNP are a current mirror, but there is 22kΩ in between which drops a bit of voltage due to PNP base current, so PNP collector current may actually be less than 10kΩ resistor current. Anyway, that gets multiplied by NPN β and causes some voltage to appear across the LEDs and 47Ω.

Now, the stupid capacitor. So NPN collector voltage is going to be noisy as hell and this couples through the cap to PNP base. Any negative edge at NPN collector pulls PNP base down, pushing more current into PNP and then NPN. Positive feedback occurs and the NPN quickly turns on, drawing extra PNP base current through 220kΩ and the cap. Until the cap charges, because then the extra PNP base current disappears, NPN collector voltage goes up and the same feedback process turns off both transistors very fast.

Then the cap discharges through 22kΩ. Rinse, repeat.

[james_s]

I'm pretty sure that "invisible" LED is actually a phototransistor to turn the light off in the daytime, I don't think I've ever seen one of these warning lights that didn't have one. Did you try it in the dark? It may have been working fine to start with.

The older lights like this used a small incandescent lamp producing brief flashes, they ran off a pair of 6V lantern batteries in parallel and would last quite a long time.

[magic]

Right.
I can't image what the LED could be for, but a photodiode/phototranny in its place could short out PNP base and disable the circuit.
Only the 10kΩ would be slowly draining the battery.

[themadhippy]

Meh, it's a simple thing.

The earlier version with an incandescent lamp was even simpler,from memory 1 transistor 1 capacitor and a resistor or 2

they ran off a pair of 6V lantern batteries in parallel and would last quite a long time.

which made them attractive to  nefarious youths,and after  said youth got a bollocking for nicking them he was amused to discover  his father and  mates used to "borrow" the original oil filled lamps  for there dens

[james_s]

The old ones I remember had two transistors. I have a couple of schematics somebody reverse engineered from them, I don't remember where I got these but they aren't mine.

[Zero999]

I'm surprised they still use discrete components. Back in the late 90s, I found part of an old one, on the roadside, which looked like it had been run over. It was just the smashed lamp head. The battery compartment had broken off. I took it to bits and found a single circuit board, with only three parts on it: a through hole amber LED, a surface mount resistor and a chip on board IC, containing the photosensor, covered in transparent silicone.

[james_s]

I'd guess it's one of those things they designed in the early 70s and just never bothered to update, the entire PCB probably costs $1 or less to produce depending on quantity. I'm not sure if they're still made the same way now though, the newest ones I've ever seen inside were probably made in the early 80s. At some point they started using LEDs instead of an incandescent lamp but I don't know if the circuit changed.

[compet17]

Thanks alot for the answers.
It is possible that the "LED" was actually a photo-diode. It looked like an ordinary 3mm LED with a transparent case, but it had an unusually large chip inside. I thought it was broken.... but after re-thinking, that might have been the cause why the lamp didn't work when it was bright.
I replaced the "LED" with a red LED and also the capacitor and the lamp worked again. Now even on the brightest day.

Maybe I should swap it back to the original.

:-) Chris

[james_s]

Yes I very much suspect it was a photodiode or phototransistor, I have seen both in standard 3mm and 5mm LED style packages.