Astable then steady

I try to build an auxiliary LED brake light driver to my bike using a basic 555 astable circuit. I want it to flash 3-4 times in the first second when the brake applied then hold a steady light until I release the brake. Any idea how to achive this?
jeno2

Use a Hex Schmitt trigger IC. Then you can pulse the LED very quickly within the "flashing" and "steady ON state", then keep it on while the brake is applied. Or you can leave out the PWM feature.

A simple, analog way to do this would be to use an LM3914 Dot/Bar Display driver.

Set the 3914 is set to operate in the Dot mode (only 1 output lit at a time).

The SIG input to the 3914 is a resistor with a capacitor to ground to provide a ramp voltage when power is applied. As the input ramps up the outputs will sequentially turn on.

Connect the LED in parallel to every other output (i.e. connections for LED 1,3,5,7,9,10) which will cause the LED to turn on and off 5 times and then say lit at the end.

The time constant of the input RC is selected to give the desired flashing time (R x C probably around 1 second).

The voltage to the RC input should be the same as the RHI voltage (typically REF OUT). Thus at the end of the input ramp the last LED driver should stay on and keep the LED lit.

Because the input is a exponential ramp from the RC circuit the LED blinking will be more rapid at the start then at the end, but I would think that would be acceptable. If you want a uniform blink rate then you would need to add a constant current source to charge the capacitor, which would give a linear ramp, and a constant blink rate.

The circuit requires 1 capacitor and 2 resistors in addition to the LM3914. The 3914 has a built-in constant current drive for the LED.

This works too.

Ken

This is really easy to do with a 4017 counter.

For 3 flashes the outputs 1-10 would go like...

1 n/c
2 ON
3 n/c
4 ON
5 n/c
6 ON
7 n/c
8 ON and Clock Enable

Have outputs 2, 4, 6 & 8 tied together (with OR gates) so when either of them go high the brake light is on. Output 8 also goes back to the clock enable. When output 8 goes high, the clock enable will prevent the 4017 from counting and output 8 will stay on (your brake light stays on).

The reset pin of the 4017 connects to your brake light switch through an invertor. When the brake is off, the invertor holds the 4017 reset high which prevents it from counting and holds output 1 high (which isn't connected anyway).

When the brake switch is on, the invertor makes the reset pin low which then allows the 4017 to count. It runs through the sequence of flashes and stops on output 8 until you release the brake.

Use a 555 for the clock and play with the frequency until it looks good.

This circuit just needs 7 parts:

Slick!

Ken

Thank You

Thanks guys,
I have to admit that my knowledge is much more limited than I could understand some of the suggestions. An other factor was that I did not want to order the parts. That is why I was thinking using 555 IC-s in the first place.
I ended up with a 556 IC bought from the Radio Shack around the corner (just like the remaining 7 pieces)
My idea was to use half of the IC as a square wave generator for the flashing, and the second half in “monostable” mode automatically triggered at start-up. When the monostable half stops (puts output to low) in about a second, it resets the flash generator in a high output stage, so my LED stay on.
BTW it works, although I still have to play around with the R, C values.
Thanks again.
Jeno