Circuit Notebook 81 - A Simple Transmit/Receive Sequencer for ATV

Where it is necessary for the transmitter and the receiver to share the same aerial then some form of coaxial changeover relay is required. The powering of different types of coax relays was discussed in Circuit Notebook 78 (CQ-TV 199).

The coax relay must only be operated and the aerial changed over when the TX is off and the TX must not be operated if the aerial is not connected to it, otherwise damage may occur.

The correct sequence is:

The circuit of the sequencer is shown in Fig 1. It has two delay circuits. U1:B delays the turn on of the TX (RLA) until the aerial relay has had time to change over to transmit, and U1:A delays the change over of the aerial relay (RLB) to receive until the TX has turned off. The relay drive waveforms are shown in Fig.2. The upper trace is at U1 pin 3 (RLB - Aerial) and the lower trace at U1 pin 8 (RLA - TX). The oscilloscope recording shows the sequence from receive to transmit and back to receive. The delay between RLA & RLB operation is measured as 1.5 seconds, but this can be altered, by changing the values of C2 & C3.

The 74HC132 (U1) is a Quad 2-Input Nand Schmitt Trigger device. The Schmitt trigger operation provides a clean output signal from a slowly changing input signal. The 74HC (high speed CMOS) version was chosen because of its high output current capability, making it suitable for driving the following transistors. It operates from a 5 V supply provided by the 7805 regulator U2.

The circuit associated with Q2 and Q3 has been described in Circuit Notebook 78 and allows a (lower cost) 28 V relay to be used from a 12 V supply.

The Sequencer was built on Veroboard and is shown in Fig. 3.


Data Sheet PC74HC/HCT132, Mullard Technical Handbook 4, 1986, Part 5. High Speed CMOC Logic HC/HCT family

Using Coax Relays with High Voltage coils, John Lawrence GW3JGA CQ-TV 199, page 43


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