A relay sequencer is used in a radio transverter and is use to activate particular stages in order after short delay. This project describes how to build this sequencer. It has the following features.

• Four negative sinked delayed output so higher voltages can be switched.

• Adjustable delay (seconds to ms).

• Input / output switching for TX or RX IF stages from radio.

• Activation on PTT through +8VDC on the radio RF input or external +ve voltage control.

• 50 ohm dummy load for radio RF signal.

• Adjustable RF level for TX IF stages.

• 12VDC operating voltage.

The relay sequencer is spilt in to two parts. The first is the power take off stage.

This stage is used to switch the output / input of the driving radio to the corresponding RX IF output or the TX IF input depending the current state of the relay sequencer.  

When the sequencer is not activated (The radio is receiving) K1 is in its N\C position, suppling DC voltage to L1 forward biasing D6, D7 and then flowing through R13 and R14 (As shown in blue arrows). C3 and C4 are used to block the DC. Once D6, D7 are conducting the RF signal is able to pass from the RX IF port to the radio input / output port (As shown in red).

When the sequencer is activated (The radio is transmitting) K1 is in its N/O position suppling DC voltage to L2  forward biasing D14, D15 and then flowing through R29 and R30. Once D14, D15 are conducting the RF signal is able to pass from the radio output port to the TX IF port (As shown in green).

The second half of the circuit is the relay sequencer. Once activated the four output transistors will start to turn on in order after a short adjustable delay. The circuit in used the set up below.

On activation Delay 1 is used to switch the RF path from the radio to the TX IF path and disconnect the RX IF path from the radio.

• Delay 2 is used to switch off any amplifying stages in the RX path.

• Delay 3 is used to switch over the output relay to connect the antenna to the TX path.

• Finally Delay 4 is use to turn on the TX power amplifier.

• Once the circuit has de-activated the output will turn off in the order they turned on.

The circuit works by using the time it take C6 to charge up and the time it takes to discharge. As the circuit is activated the voltage across C6 increases over time as shown below.

The capacitor takes 5 time consents to change up to full. 1T = R * C (C6* R18+R22). The 4 op-amps are setup to turn on at different voltages as C6 charges up.

• Delay 1 at 3.59v but as is wired in the to input, this op-amp turns on instantaneous.

• Delay 2 at 6.08v (50.6%).

• Delay 3 at 8.56v (71.3%).

• Delay 4 at 11.05v (92.1%).

When the circuit is de-activated the four op-amps will turn off is the opposite order as the turned on.

The op-amps are used to drive four transistors in sink mode so a higher switching voltage can be used. The transistors are able sink a MAX 1.5A at 80V and four LEDs are used for indication of the outputs are active.