555 Timer Monostable Mode Self Triggering

Question

I'm working on a simple project with 555 timer. I'm trying to generate a high-state output for 10s when Vcc is applied to the circuit(It is initially off). However, I have not managed to operate the timer correctly due to self-triggering problems.

^{simulate this circuit – Schematic created using CircuitLab}

It was my first attempt to self-trigger problem according to some examples I found on the internet. Trigger is oscillating between 4V-8V in this configuration which leads to repeated negative-edge transition on the trigger pin; therefore, output never falls off zero volt. Secondly, I've tried a zener in parallel with 470uF capacitor to regulate voltage at trigger pin to prevent another hi-lo transition; but, it has failed again. After 3-4 hours of research, I haven't found a solution yet. In short, the problem is to generate a self-trigger mechanism to 555 timer without any external interventions except turning on/off Vcc. Any idea or explanation will be very helpful.

Thanks.

edit: As Dave suggested, I connected an RC network for triggering purpose and got output as I desire. Following graph is the transient response of that configuration.

Blue: Out Green: Trigger Red: Threshold

I didn't get a point here. As I know, 555 timer set its output high in monostable mode when trigger has a hi-lo transition. What am I missing?

Answer 1

You can't use the main timing resistor and capacitor to generate your trigger signal. As you discovered, this is exactly equivalent to the astable configuration.

Instead, you need to provide a separate resistor (to Vcc) and capacitor (to ground) for the Trigger pin. The time constant for this pair should be short relative to the main timing period, but long relative to the risetime of the power supply.

These components will hold the trigger low while the chip powers up, but then allow it to go high a short time later. Once the timer has timed out, the main timing capacitor will be discharged, but the trigger capacitor will not.

Answer 2

Triggering when power is applied is a bit of a thorny problem- especially if you want it to be bulletproof for different rates of rise of the power supply or to trigger if there is a blip in the supply.

One method that works very well, but may well be overkill for your situation, is to use a supervisory chip such as the LTC2912 (it has an internal shunt regulator so it will operate from your 12V with a series resistor). It has a timer in it so it could replace the 555 as well (or not, your choice), with a 1uF timing capacitor. Supervisory chips have an absolute voltage reference internally and are designed to operate at a low voltage so they can be pretty much bulletproof if you set the threshold right.

If the turn-on is well defined and sharp every time, Dave's suggestion is a good one, and is inexpensive. If you parallel the trigger charge resistor with a reverse-biased diode (eg. 1N4148) the blip performance will be improved.