One transistor switch

A digital switch consisting of only one transistor and 2 resistors. It can be a replacement for e.g. in some cases rather complex digital circuitry to send through or block digital data from one point to another.

Circuit diagram


The "switch" can be closed (data throughput from "Input" to "Output" enabled) by putting a "1" to the "Enable" connection. It can be opened (data throughput from "Input" to "Output" disabled) by putting a "0" to the "Enable" connection.
The "switch" is not an ideal one, it is a "one direction" switch, hence it has an input and an output, which can not be interchanged. If the switch is open (not enabled), the output is always "1" (unless some other component on that output connection pulls it low of course). Outputs of several switches can be tied together, in this case resistor R1 has to be present only once. Tied together outputs make a "wired and", the common output is high if all inputs of enabled switches are high.
An example of its usage can be found in e.g. TelnetToRs232.GIF .