Odd useful bits of circuitry.

The Servos used for remote control model aeroplanes and helicopters are an obvious choice for controlling equipment in high altitude payloads (such as camera control and steerable recovery systems).

Servo typically have 3 wire interface:-

• Ground (typically Black)
• Power (4.8V) (typically Red)
• Control (typically White)

Some connectors have a key-way next to the control wire.

The control signal takes the form of a pulse train – nominally with a period of 20ms – but anything in the range of 18 to 25ms should be expected to work. The pulse length is varied to set the position of the servo – 1ms equates to a position at one end of travel – 2ms to the other end of travel – with 1.5ms being a central position.

The Rate gyros used in model helicopters conform to the same interface as RC servos. Normally the control for the RC system is passed though the Rate Gyro before being applied to the servo – that way the rate gyro can modify the pulse width (typically to stabilise yaw in a model helicopter)

Op-to couplers are a great way to interface micros to odd bits of equipment that would normally need mechanical switches to operate them. Their big advantage is that the provide electrical isolation (normally a few thousand volts) between the micro and the device. This means that device can be wired totally independantly of its circuit design - for example allowing it to be run off the same battery as the micro even if the device switch circuitry does not share a common contact with the supply rails. The other advantage is that the isolation provides protection against electro-static discharge.

Opto-couplers consist of a LED (usually infrared) and a opto transistor in a single package. If current is passed through the LED its light switches the transistor on. Opto-isolators need to be wired into the device correctly in order to work. Using a meter measure the voltage across the device's switch in order to work out the polarity - and then wire as per the diagram :-

A dual opto-coupler and resistors in use to interface a camer requiring both a power on switch and a shutter trigger switch.