Electronics Components: How to Use the 555 Timer Output
The output pin (pin 3) of an electronic 555 timer circuit can be in one of two states: high and low. In the high state, the voltage at the pin is close to the supply voltage. The low state is 0 V.
There are two ways you can connect output components to the output pin: source and sink. To illustrate these two configurations, consider a configuration using an LED as the output device with a resistor included in the circuit to limit the current flow. Without the resistor, current will flow through the circuit unimpeded, which will quickly burn out the LED and probably ruin the 555 as well.
In the circuit on the left, current flows through the LED circuit when the output is high. The current flows from the output pin through the LED and resistor to ground. This output configuration is called sourcing because the 555 is the source of the current that drives the output.
In the circuit on the right, current flows through the LED circuit when the output is low. The current flows from the Vcc supply, through the LED and resistor, and into the 555 where it's internally routed to ground through pin 1. This output configuration is called sinking because the current is sent into the 555.
Whether you source or sink your output circuit depends on whether you want your output circuit to turn on when the output is high or low.
You can combine both sourcing and sinking in a single circuit. Here, two LEDs are connected to the output pin. One is sourced; the other is sunk. In this circuit, the LEDs alternately flash as the output switches from high to low. LED1 lights when the output is low, LED 2 when the output is high.
The output circuit of a 555 timer can handle as much as 200 mA of current, which is actually much more current than most integrated circuits can source or sink. If you need to drive a device that requires more than 200 mA, you can isolate the output device from the 555 by using a transistor.