4017-555 LED Flasher - Temp Effect Problem

I've soldered together two identical circuits similar to the drawing attached.

**broken link removed**

I made a few modifications to the circuit to reset the 4017 counter and 555 counting on startup - these mods work fine.

I also have the output from the 4017 counter flipping a relay using a npn transistor.

Both circuits flash at the same rate - at room temperature.

Here's my problem...

If I add a little heat to one of the circuits - using a hair dryer - just enough to warm the circuit board - the leds start flashing SLOWER.

I'm using two 10 uF tantalum capacitors.

Using the heat from the soldering iron I found that polyester capacitors - when heated - sped up the circuit.

I REALLY need to figure out how to get both circuits running at the same rate under different temperatures - say 0F to 100F - for an automotive application.

Any ideas?

Use capacitors with a small temperature coefficient. Only the good caps list their tempco. When a capacitor has a large tempco; they don't tell you.

Using carbon film resistors in a timer circuit makes it extremely inaccurate (temperature drift) because of the high positive temperature coefficient of the resistors.

What kind of resistors did you use?

That oscillator is a bit hinky, vary up it's design a bit, there are lots of ways to make a 555 an oscillator.

For example...



This design has a problem yours does, you really need a small resistor in series with Ra, otherwise when pin 7 shorts (a normal function) you can burn the 555 out.