A computer’s serial port is a convenient source of low-speed dc control signals that are easy to set from software. For example, the RTS (request-to-send) line outputs about +9 V when “on” and -9 V when “off.” This idea shows how to use this signal to control the shutter of a Canon EOS 40D camera for a computercontrolled time exposure.
An ordinary cable release for this camera contains two switches, mechanically linked so that as you press the button, first the “focus” switch and then the “expose” switch closes (Fig. 1). On the EOS 40D, long exposures on “Bulb” require both switches to be held closed, but isolated from each other when open so that the button on the camera will work correctly.
The computer-controlled cable release uses a resistor and two 2N7000 MOSFETs connected to the serial port (Fig. 2). When the RTS signal goes high, both MOSFETs conduct and the shutter opens. When RTS goes low, the shutter closes. The resistor dissipates static electricity and prevents erratic operation when the serial port isn’t connected.
Similar circuits can perform other switching functions under computer control. The big advantage of the 2N7000 is that its gate can withstand ±30 V, well beyond the voltage range that the serial port can output. Thus, there’s no need for a clamping diode or voltage divider.