Help needed with simple circuit design

Hi all,

I am building a replacement flasher unit for an automotive turn signal circuit. [The original bulbs are to be replaced with some prototype LED setups, which cause incorrect flash rates. Now that I'm down this path I would like to get my design working!] I built a circuit that toggles a relay at the desired rate... which works great on the test bench.

During fitting, I found that the in-car circuit is different than I first believed. The flasher unit has power with the ignition on, and the appropriate bulb circuit is connected by the indicator switch, completing the circuit to earth. I originally believed that it was the positive that was switched, and my circuit uses this and a separate earth. In this configuration the flasher circuit is always active with the ignition switch on... :(

I have made a quick circuit diagram of the indicator circuit - please see: http://202.74.169.80/images/circuit.gif

With reference to the circuit image - The relay shorts "A" and "B" to activate the turn signals. My flasher circuit takes power from "A" and "C" [which is no longer appropriate].

Any suggestions as to how I can power my circuit so it activates with the turn signal selector? It only requires ~3V and maybe 100mA.

Nick

Interesting... have you heard of FMVSS 108, ECE-R 48-01, or EEC 76/756?

If you have to do something in a hurry, go down to the auto parts store and buy an "electronic" turn signal flasher. Here, the regular thermal type costs about US$2, and the electronic type costs about US$10. The difference is that the electronic type will work over a much wider range of loads than the thermal type will.

Pretty standard in most cars.

What's that thing between point B and the indicator selector?

One idea is to replace the relay with an appropriately rated FET. It can sit there and switch on and off all it likes, all the time, and you won't have to hear the clicking. When the indicator switch is turned on, the switched current will go to the lamps like normal. If you want to have the clicking noise to remind you that the indicators are on, wire in a relay coil and leave its contacts open, like this...

2 x LED o------*----->|---ground A---FET---B---*--- | o----*-|----->|---ground indicator | | switch | +-->|--+ | *----*-relay coil-*--ground +---->|--+ | | 2 x 1N4002 +-----|

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Hi Mark,

Yes, that was my first idea... but when the relay shorts "A" and "B" there is no power to the flasher circuit and the relay drops out... providing power to the circuit again and pulling the relay in... which shorts the power to the circuit and.... you get the idea :)

I was tempted to get an electronic flasher unit, but the circuit uses the first PIC I have ever programmed (exactly halved the component count from the 555 version) and I would hate to admit defeat ;)

--Nick

Hi Matt,

The thing between point B and the selector is an in-dash indicator lamp. Yes, I am familiar with the appropriate regulations - I am in Western Australia which has similar (although more relaxed) requirements.

The (pedantic?) issue with having the circuit running the whole time is that the flasher could be anywhere in the ON/OFF cycle when switched in. It would be much more practical if the signal lamps illuminate immediately when the selector is moved ie. start the ON/OFF cycle.

Your second circuit idea looks promising... I will try and implement something using this approach.

--Nick

see:

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I used a relay with a 600-ohm coil and a red flashing LED between the coil and power.

Low parts count....

You've got a PIC? Then you can do anything you want. Put a resistor, say

10K, right from A to B, http://202.74.169.80/images/circuit.gif and sense the voltage at B. If it's near ground, turn on the mongo switch from A to B to turn on the light. After the right delay, turn off the switch, wait an appropriate "off-time", and check point B again. If it's low, start the cycle again; if it's high, idle until it goes low again. Don't look at the voltage on "B" while the swtiching element is on. ;-)

Have Fun! Rich

And for the production unit, when the signals are first turned on, start a one-minute timer. If after one minute, it's still flashing (i.e., the lever hasn't been reset), start a very loud annoying beep to remind the weinerheads to turn off their signal on the freeway. ;-)

Cheers! Rich

"Nick" schreef in bericht news: snipped-for-privacy@asteroid.globaldial.com...

Are you sure your LEDs provide enough light? Although LEDs tend to give more light for the Watt then incandescent bulbs, they are not that much better. Your LED-load should not be much less then the original lightbulb if it provides the same amount of light.

Another thing you may take into account is the warning that comes from the ticking of the thermal flasher. If the load changes due to a failing bulb it can be heard immediately.

petrus bitbyter

Hi Nick,

Since you are in WA have you thought about an an Australian made and approved automotive electronic flasher already on the market here which are designed for LED lamps and which will provide the functions required.

Be aware that many so called elctronic flashers are deliberately load sensitive to cater for normal bulbs. You need a unit which is not load sensitive such as the 10A and 20A units on this page