An easily built project for interfacing with most transceivers

by Chris B. Sakkas KB8ITU

The Simple Simplex Repeater is a low-cost, easy-to-huild project that, when interfaced with a transceiver acts as a limited simplex repeater Unlike most repeaters, a simplex repeater operates on a single frequency by recording the incoming transmission and playing back this recording on the same frequency. Operators who would otherwise not he ahle to contact each other directly can use a simplex repeater operated by another party to communicate. The Simple Simplex Repeater is handy in these situations and tor use during emergency conditions.

The Simple Simplex Repeater is easy to interface to most transceivers, and can be built using readily available parts. Only three ICs and a handful of other components are needed to build the project The core of the project is the rSDIOOOA Voice Record and Playback IC. available from Radio Shack, that stores up to 20 seconds of audio irom an incoming transmission. When the incoming transmission has ended, the repealer keys the transceiver and plays the recorded transmission.


Figure 1 shows the schematic diagram lor the Simple Simplex Repeater Point J1 is the input ol the carrier-operated squelch (COS) signal from the radio. A received audio signal is said to be present when the COS signal is greater than 0.7 V. switching the collector of transistor Q2 to ground. The play/record UV^R) pin of Ul. the ISD1000A, then becomes low, placing U1 in record mode. NAND gates U3B and U3C are set up as inverters, and thus generate a high output. D flip-flop 1-4A is clocked on this positive edge, therefore making the Q output high. NAND gale U3A has hoth inputs high, therefore generating a low input to the power down (P!)) pin of Ul. The resistor R2 and capacitor C6 combination is used to generate a brief delay.

After COS drops below 0.7 V. the P/*R pin of Ul is made high, placing Ul in play mode. D Hip-flop U4B is clocked on the positive edge, making the Q output high, which ke\s the F1 I" of the radio by switching transistor Ql. l iie R3-R4-C7 combination works as a negaiive-edge triggered negative pulse, which places the output of NAND gate U3A high for a brief period to allow UI to enter playback mode.

When pla> is complete (or the audio memory is full), the end of message (*EOM) pin of Ul goes low. This clears U4A and U4B, allowing the PTT to be released and Ui to power down. The play sequence is entered if ihis state is created by a memory lull condition after COS drops below 0.7 V.

Potentiometer RI controls the u>lume of the audio out signal. A transformer can be used to match the output impedance to the radio, if desired.

U2 is a 5 V voltage regulator included in the design to allow connection to various unregulated DC supplies between the ranges of 7 V and 12 V. if. however, a regulated 5 V supply is used to provide power, U2 can be eliminated and the regulated supply can he connected lo the points in the diagram.


The Simple Simplex Repeater can be interfaced easily to most radios. Six in-

Parts List for the Simple Simplex Repeater


U1-ISD1000A Voice Record/Playback IC (RS 276-1325* U2—7805 5 V voltage regulator U3—74L300 quad NAND oate IC U4-74LS74 dual D flip flop IC □V Q2-2N3904 NPN Iransislors


{All fixed resistors are 1/4-watt, 5% units)

R1—IK ohm potentiometer

R2 -15K ohm


Cl, C2. C3, OA C5h C6 -0 22 mF capacitor iRS 272-1070) C7—220-|iF electrolytic capacitor (RS 272-1029)

Additional Parts and Materials

Connectors solder, bread boarding maienal, power supply




Connect to carrier-operated squelch of radio. Must provide >0 7 V if a carrier is present and <0 7 V it no carrier is present.


Unregulated DC input from 7 to 12 V.

J3—Audio Out

Connect to audio input of radio and adjust R1 to suitable volume level


Connect to push-to-talk of radio to control transmitting This point is grounded to key the transmitter.

J5—Audio In

Connect to audio output of radio. The input impedance ot this point is 2.7k ohm and the maximum drive level is 50 mV p-pT


Connect to ground of the radio.

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