A 555 IC Timing Chip is a widespread electronic component, used mainly in timing circuits and to debounce an input signal. There are two main configurations.
Astable
An astable setup has the output changing constantly between on and off. Pin configuration:
- Linked to 0V.
- 'Trigger'. Linked to pin 6.
- Output. Commonly linked to a BC108 Transistor, the activation of which turns on a relay.
- Linked to the positive rail.
- Unused.
- Linked to 2, R2, and the positive leg of C1.
- Linked to the other side of R2, and to R1.
- Linked to positive rail.
Additional components in Astable Configuration:
- R1 - connected to positive rail, to R2 and to pin 7.
- R2 - connected to R1, C1, pin 6 and 7.
- C1 - connected to R2, pin 6 and 0V.
The frequency of the output changing can be worked out with the following formula: f (Hz) = 1.44/(((2*R2)+R1)*C1). Please note that the 'on' and 'off' times will be microscopically different.
Monostable
This configuration, when triggered, goes 'on' for a certain length of time, before going 'off' again. Once 'off', it will remain off until triggered again. This can be used to replace a Schmitt Trigger in debouncing a signal. Pin configuration:
- Linked to 0V.
- 'Trigger'. Linked to SW1.
- Output. Commonly linked to a BC108 Transistor, the activation of which turns on a relay.
- Linked to the positive rail.
- Unused.
- Linked to 7.
- Linked to R1, pin 6 and C1.
- Linked to positive rail.
Additional components in Monostable Configuration:
- R1 - connected to positive rail, pin 7 and C1.
- C1 - connected to R1, pin 7 and 0V.
- SW1 - connected to pin 2.
The time in seconds of the output remaining on can be worked out with the following formula: t (s) = 1.1*R1*C1.