# 555 Timer, Astable or Monostable?

Discussion in 'General Electronics Chat' started by surina, Jan 13, 2016.

1. ### surina Thread Starter New Member

Nov 11, 2015
10
0
Hi,

I want to produce a circuit with an output of 2 Hz with duration of 0.5ms. My pwm will be 0.5ms and the duty cycle is 0.01%. I have made one circuit with arrangement of astable to make it operates for 2 Hz but I am confuse on how I am going to make it fire at 0.5ms pwm? Do i need to connect with another monostable circuit to have the pwm as 0.5ms? Here i attached my circuit connection for astable.

2. ### hp1729 Well-Known Member

Nov 23, 2015
2,087
232
A positive output pulse of 0.5 ms, 0.01% duty cycle? sounds difficult. R2 controls discharge time and has to be large for a long discharge time. But this resistor is also used in the charge cycle so a short high time is killed.
Two possibilities. Small R2, large R1, and invert the output with an op amp or transistor.
Or two monostable 555s that trigger one another.
Do either of these fit what you need? A 0.5 ms positive pulse at 2 Hz?

• ###### Design 566 2 Hz oscillator.pdf
File size:
46.4 KB
Views:
5
Last edited: Jan 14, 2016
absf likes this.
3. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
You can tune the duty cycle of a 555 astable by using the output on pin three as feedback to the pin two/six node. Connect the feedback through a variable resistor, pin three connected to the wiper, and the ends connnected through diodes of opposite polarity so that the charge/discharge of C is steered through different end connections of the resistor. Frequency is determined by the total fixed resistance of the feedback resistor and capacitor, and duty cycle will depend on the ratio of resistances between the wiper and each end. Also, you can use two fixed resistors in place of the variable. The same idea will apply for the fixed resistors.

Sorry, I don't have a drawing. Hope you understood this. This circuit topography can be found using a good key word search, and hopefully, my description will help you to recognize it. Otherwise, I'll try to find a drawing.