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As I understand it, there are many ways to combine two resistors and a capacitor to give the same time characteristics in a astable 555 setup.

My question is which is better, a larger capacitor or a smaller one? and does it effect power consumption?

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Well a larger capacitor needs more energy stored to trigger the threshold pin, this energy is just dumped when the output is low. A lower valued capacitor needs larger resistors for the same timing but waste less power. I'd say make the capacitor as small as possible while keeping the circuit functional to save power. Eventually the resistor needed will be too large compared to the 555 input resistance and you'll hit your timing limit for the small value capacitor.


An input pin on any device needs to have a large input resistance as to not disturb the point it is measuring, you can visualize this as a really large resistor connected to the pin and ground. This means that if you connect it to a point in the circuit with a high resistance value before it you'll get a voltage divider at that point. If you keep your resistors small enough this ought never to be a problem. Going higher will tend to degrade performance and could cause the unit from working completely.

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For example the resistors above should be chosen to pass at least a certain amount of current to the 555. Examining the data sheet gives the minimum values:

Trigger current (pin 2): 1uA Reset current (pin 4): 0.5mA Threshold current (pin 6): 0.25uA

The values of your resistors will thus depend on the configuration of your circuit, operating voltage and these minimum currents. Don't worry too much about this, play around with your circuit, keep increasing the resistance for a specific capacitor value and when the circuit doesn't work as expected anymore you know you have to increase your capacitor value instead to get a longer delay. Or, if you're up for it, try to calculate the best values for your configuration.

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  • \$\begingroup\$ Thank you. Could you please explain what you mean by the resistance being too large compared to the 555 input resistance? \$\endgroup\$ – seininn Oct 17 '12 at 13:24

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