Electronical switch with HEF4538


With this electronical switch it is possible to release a camera with a radio control system by switching relay contacts. To save energy this circuit is useful to switch the power supply of a video transmitter to control the direction of the camera, too. The switch is realized with a relay to prevent interferences between the circuits of the camera and the radio control system.

Functional Description
The electronical swith is made by 2 timers in the integrated circuit HEF4538 (TIMER1 and TIMER2). TIMER1 starts a reference signal on the leading edge of the pulse from the RC-receiver. At the end of this reference signal (about 1.5msec), the inputs of TIMER2 do the compare with the signal from the receiver and the signal from the reference timer. If the signal from the receiver is finished the TIMER2 starts with a time of about 30msec. This time is for waiting until the next trigger of the next pulse and trailing edge of the TIMER1. The output of TIMER2 drives a relay. This relay will switch the contacts until the signal from the receiver is longer as the reference signal.


Aligning the function
Using a radio control channel with joystick is the best way to test and align the function of this circuit. At first place the joy stick in the center position. Fix a measuring instrument for resistors to the contacts of the relay switch, to see the work of the relay. Usually the relay should switch if the joy stick is moved towards one direction. The other direction should have no function. With the potentiometer you set the reference time to compare the signal from the radio control receiver. In one maximum position of the potentiometer the relay will never switch. In the other maximum position of the potentiometer the relay switch is always on, if there is a signal from the radio control receiver. Now, turn the potentiometer to the other direction until the relay will switch off. With small changes on the potentiometer position you can adjust the right point for the position of the joy stick on the radio control transmitter.

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©2015 Harald Prinzler