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.