How much trouble making an LED flash ?

Well it all started a month or so ago. I had a call from one of my Distributors saying he had a little project for me.

Design spec was to make an LED flash. Not a problem I thought and went over for a meeting.

LED was initially a 1W Luxeon - still not a problem. Housing for project was smaller than Luxeon - ok - problems start

Maximum size of PCB which had to include the batteries (3 button cells), a switch, the flashing circuitry and the LED was now restricted to 40mm by 19mm - by the time I mounted 3 x SR41 cells on with the holders there wasn't much room left.

Found a 5mm LED which gave the correct viewing angle for the project and knocked up a flip flop out of transistors. Prototype looked feasible so we went out and bought some dedicated MT flasher ICs in a TO92 package. With a on/off ratio of 7:1 the batteries should last around 6-8 hours.

Knocked up some "semi-final" prototypes and my Distributor presented them to his customer who didn't like the flash rate. He wanted 5 seconds on and one second off with the option to change it. Bang go the nice easy MT chips.

Now - how to get 6-8 hours usage from 3 x SR41 cells (around 30mah) driving a 25ma white LED .......

Oh did I mention it had to fit in a tube so height was severely restricted ? The three button cells only just fit into the tube on one side of the board.

Currently working on using a 10F200 with the provision of PWM to try and maximise the battery life, bi-colour/tri-colour LED driving, automatic power off, various flashing patterns and if I have the space I'll try and make the flaming thing play Dixie as well

Don't you just love it when the goalposts keep moving

Since the 1.55V SR41 is only about 45mAH you've got your work cut out for you.

Well its all money at the end of the day.

Just finished the coding for a test unit tomorrow - will have to solder a SOT23 18F200 into an 8 pin DIP socket and have a play with it to see how well it works.

I mght have to program an "Easter Egg" into it so it flashes rude messages via morse code or something

picbits said:

I mght have to program an "Easter Egg" into it so it flashes rude messages via morse code or something

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Now thats a good idea, would be fun to modify an LED keyfob and see if anyone notices...

Well the prototype has been running continously in 1/7th duty PWM mode for the past 4 hours and is still going ....

Those 10F200 in surface mount are a pig to solder by hand.

Trying to get a 10F to do multitasking and PWM with only 256 bytes of memory, no interrupts and only one 8 bit timer was rather fun. I forgot how spoilt I was with the 18F I ususally use.

Well I was hoping to get between 6 and 8 hours out of a set of batteries.

Friday it ran from 4pm to 11pm - 7 hours
Saturday it ran from 9:15 to 12:15 - 3 hours
Saturday evening it ran from 5pm to 11pm - 6 hours
Today its been going since 9:15 am and is still running getting quite dim now though so I'd day another 9 hours before it showed real signs of dimming.

Thats a total of 25 hours running a 25ma LED and a 4Mhz PIC micropocessor from a set of three 38mAh SR41 cells.

Cant really complain at that - even the missus is impressed and it takes a lot to impress her if its got wires on it lmao.

How bout a picture?

Krumlink said:

How bout a picture?

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I would do if it was my own product but I'm working on behalf of a customer and this is going to be a commercial product very shortly so they now own the IP for item

Just turned it back on this morning and the batteries have recovered enough to flash it back at full power for a bit.

That makes sense, cant believe they want all that for a flashing led.

Well the original spec was just to make it flash. Things went downhill rapidly from there

When you're asked to make an LED flash for 5 seconds on and 1 second off its not a problem. When you're asked to make it last for 8+ hours from a 38mAh power source things get much harder. Its even more fun when you have to fit it in a casing that you can only just get the three SR41 cells in ..........

How many did they want of this product?

I think that they are purposely giving you a hard time

Not bothered how many they want - thats up to my customers customer lol. I charged by the hour and got paid by the hour. My side of the contract is now finished until they decide to alter the parameters again at which time I'll charge them again for however long the work takes

There have been numbers of thousands banded around but as I said - thats up to them.

how many hours

Just waiting for Vista to recognise my camera (takes around 15 minutes !!!) and I'll post a couple of pictures of it out of its case.

Krumlink said:

how many hours

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Charged for 6 hours - took me around 10 hours.

Here we go - apologies for the quality but its a cheap camera