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Relay Sequencer Board
author: Ben Hennessy  - mail


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.

Circuit Diagram

Download Schematic in PDF format

Parts List

R1       1K
R2       1.8K   
R4       10K    
R6       4.7K
R7       4.7K
R8       1K      
R10     10K
R11     4.7K
R12     1K
R13     1K
R14     1K
R16     10K
R17     4.7K
R18     4.7K
R19     4.7K
R20     100
R21     1K
R22     1K ADJ
R23     100
R25     10K
R26     120
R27     6.8K
R28     500 ADJ
R29     1K
R30     1K
C1       10uF
C2       10uF
C3       1nF
C4       1nF
C5       1nF
C6       1nF
C7       1nF
C6       4.7uF or 2.2uF
L1        10mH
L2        10mH
L3        10mH
D1       LED
D2       1N4148
D3       1N4148
D4       LED
D5       1N4148
D6       1N4148
D7       1N4148
D8       LED
D9       1N4148
D10     LED
D11     1N4148
D12     1N4148
D13     1N4148
D14     1N4148
D15     1N4148
D16     5V1
D17     5V1
D18     5V1
D19     5V1
Q1       BD139
Q2       Bc547
Q3       BD139
Q4       BD139
Q5       BD139
U1       LM7812
U2       MC455
U3       MC455
K1       Relay SPDT




Download this circuit in DOC


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