Hi guys. I am wanting to power a speaker with 2 astable timers. I want the first timer to occilate slowly at about 3Hz, and the second timer to be at 50-100Hz. Pretty much it will be an alarm/buzzer. I've tested it and it works fine, but the only problem is that when I first apply power to it the first timer's first ocillation lasts longer than than its other ocillations. I know the reason it does this and it's because the capacitor has to start charging from 0 volts to 2/3Vcc(When first powered on) instead of 1/3Vcc-2/3Vcc so basically the first occilation is 2 times longer. Anyway, the point is that I want my alarm to be consistent so is there anyway I can make the first oxcilation consistent with the rest?
555 Astable Alarm Help
- Thread starter Matthew464
- Start date
AlbertHall
- Joined Jun 4, 2014
- 12,347
See figure 3 here: http://www.davidbridgen.com/555.htm
You could try using a circuit that would rapidly charge the capacitor to just below 1/3 of Vcc. Have a potential divider (Using low enough values to charge the capacitor quickly) that gave a voltage of a little less than Vcc/3 + 0.6 V and connect its output to the capacitor via a silicon diode.(Positive to the capacitor) The first pulse would be a little longer than normal but not twice as long.
Les.
Les.
Put a delay on the reset pin of the 100hz oscillator using a resistor and capacitor, (resistor to positive,capacitor to negative supply,) this will keep the output low until the 3hz oscillator gets upto speed, values of 470K and 1uF will give approx half a second.
The problem with that is the reset pin has a low reset voltage (basically one base-emitter drop) with a significant output current, so the delay time will be much shorter than you stated.
Below is the LTspice simulation of a 555 astable circuit with an added transistor to provide the hold-off.
The first cycle of the 100Hz oscillation is longer but I wouldn't expect that to audible.
