PTT Delay Circuit
By Kevin Custer  W3KKC

Concept: I use several Advanced Computer Controls, (ACC), RC-85, and RC-96 repeater controllers in my linked repeater system called "The Target Link System"

When linking two repeaters together, sometimes it is nice to get rid of all locally produced courtesy tones. When linking or remoting with the above controllers, and the link or remote transmitter is enabled, the controller produces a "link ptt" courtesy tone before sending the regular selected, one of  ten, courtesy tone. The regular courtesy tone can be selected for no tone with the  (control op prefix + 70) command, however  the link ptt courtesy tone cannot be disabled by a control op command.

If  "Zero Hang Time" is enabled, (control op prefix + 66) command, when link or remote base is on, and in transmit, the controller will not produce ANY courtesy tone. This is great, but there's a problem. When a link or remote transceiver switches from transmit to receive, there is a "transition period" between these two states that is neither transmit nor receive. This is especially true when using the FC-900, and ICOM band units, because of the serial tuning. The repeater transmitter ptt follows the main receiver cos/pl configuration, and link receiver cos, the transition period will cause the main repeater transmitter to briefly un-key until the controller has been sent "link cos". Thus the need for a "Ptt Delay Circuit". This situation does not happen when not in zero hang time because of the hang timer, unless hang timer is set to zero or very short. Also this situation does not happen when two repeaters are "hardwire linked" at the same site. Hardwire linked means not using radios to link.

Description: The delay circuit is inserted between the controller and transmitter to provide a set-able time delay keeping the transmitter keyed when using "zero hang time" and linking to another machine. To use this circuit, the controller's repeater ptt must be configured "high true" (dip switch 2 on) .

Benefit: Eliminate all courtesy tones when linked to provide transparent operation, and eliminate the brief un-keying when remote or link is in transition between transmit, and receive when using zero hang time mode.

Drawback: The only drawback is the added hang time may not be desirable when not linking, however you can compensate for this by decreasing your present hang timer setting, or for other reasons not foreseen here.

Construction: Construction can be simple breadboard wiring, as the circuit is simple. A reed relay is used for two reasons. One is, the time delay in this circuit is provided by charging and discharging a capacitor, and connecting the output directly to the transmitter will provide a voltage that does not snap between on and off, thus the characteristics of the relay are desirable. Second, interfacing the relay is simple, simply connect one contact of the relay to either ground or voltage depending on the signal needed to key your transmitter. I have used this circuit in several systems for years with no relay failure ever. Radio Shack sells good quality reed relays. VR 1 is a sensitivity control. Because this circuit can be used for many other uses, and not knowing what signal is available to drive this circuit, thus the need for a sensitivity, or gain control. VR 2 sets the time delay, and is used in conjunction with C2. C2 can be changed up in value to provide longer delay time, or lessened in value to do the same in time.

This circuit design is very straightforward, and reliable. Several copies, of this circuit, have been used in my system, and others for years with no failures except for lightning.

Parts List:
Q1 & Q3, 2N2222 or equivalent.
Q2 & Q4, 2N3906 or equivalent.
D1, 1N914 or equivalent.
R1 & R4, 10 Kohm
R2, 100 Kohm
R3, 47 Kohm
R5, 2.2 Kohm
VR 1 & VR 2, 47 Kohm pot.
C1, 100 pF disc ceramic.
C2, 4.7 uF tantalum, 35V.
C3, 100 uf or larger 35V electrolytic cap.
Reed relay, 12 volt reed relay.

Image of PTT Delay
This circuit was designed by my Father, Ken Custer W3WGX in the 70's
HTML Copyright© 1998 Kevin Custer W3KKC

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