Circuit Notebook 80 - Satellite Receiver Control

The Anglesey ATV Repeater GB3TM had been operating reliably, with exception of a seized cooling fan, for about 6 years. Last year several faults developed, one of which, required the replacement of the original receiver.

The new receiver is a PACE 9200 satellite receiver. This has good RF performance and at present is operating without a pre-amp. The receiver frequency and other parameters can be programmed in and stored using the remote control.

When first powered-up the receiver is in standby mode and the 'on/standby' button must be pressed for the receiver to operate. It commences operation in channel 1 and this can be programmed for the repeater input frequency.

The requirement was for an automatic switch, which on powering-up, after a mains failure, would electrically switch from standby to operate, on channel 1.

The circuit of the control switch is shown in Fig.1. It consists of two 555 timers operating as two mono-stable circuits, one triggered by the other. I am not a fan of 555 timers. They virtually short circuit the supply rails during switching transients and produce spikes on the supply, causing false triggering of other devices. The decoupling capacitor C6 is therefore an essential component needed to prevent this problem.

Circuit Operation

At power-up, the trigger input, pin 3, of IC1 is low, thus starting the delay period, which is determined by R2 and C2. D1 is provided to rapidly discharge C1 in the event of a mains glitch. The output, pin 3, of IC1 goes high and provides a delay of about 1.5 seconds.

At the end of the delay period, pin 3 goes low and the negative transition passes via C3 and R3 to the trigger input, pin 2 of IC2. A low-pass filter, formed by R3, C4 is included to eliminate spurious spikes, which appear at the output of IC1 on power-up.

The IC2 mono-stable operates for about 1 second and the output at pin 3 energises the relay for this period. D2 and D3 are fitted to prevent IC2 latching-up. The relay coil should have a resistance greater than about 200 ohms and most small 12 V relays will be suitable. The relay 'normally open' contacts are wired directly across the receiver 'on/standby' switch contacts.

The circuit was built on Veroboard and installed inside the receiver, the 0V connection is connected to chassis and the + 12V to the receiver + 12V line.

Although this circuit was built specifically for our repeater, it may have uses in the shack for booting up the station satellite receiver instead of using the remote control.

References

555 & 556 Timers, Signetics Application Booklet Signetics Corporation 1973

NE555, Data Sheet NE 555 Precision Timers Texas Instruments, Linear Circuits

Data Book, Vol. 3, Page 4-11 (1992)

Figures

Fig.1.Satellite Receiver Control Switch