Installation
C-5
Repeater Transmitter PTT
Your transmitter will require a PTT (Push-To-Talk) Signal from the controller.
When this signal becomes active, the transmitter will Key (Transmit). At this
point, it is unimportant whether your transmitter’s PTT input is low active or
high active since the controller has sense-reversal DIP switches (discussed
later). The PTT input from your transmitter should be connected to Pin #10 of
J2 (the female DB25S connector).
To get a better idea of how to interface your particular PTT input to the
controller, let’s examine the controller’s PTT output circuit. This circuit consists
of a power MOSFET connected as an open-drain driver, plus a transient
suppressor connected between the output and ground. The suppressor
removes transients above about 40V, thereby protecting the MOSFET. The
MOSFET is a large (8 Amp) device in a TO-220 package, but it was not
selected for its high current capability. Rather, it was used because of its low
ON resistance (less than 1 Ω), making it a nearly perfect switch. The
MOSFET’s drain is connected to the PTT output, and its source is grounded.
When turned ON, the MOSFET appears to be a very low resistance between
the output and ground. When turned OFF, the MOSFET appears to be a very
high impedance, isolating the output from ground. Because of these
characteristics, the PTT circuit can control transmitters with a variety of PTT
inputs, from TLL-logic-compatible to large DC relay coils.
Some transmitters, including models made by Hamtronics, MELCO (Maggiore
Electronic Laboratory), and RCA (500-and 700-series), use a powered PTT
input arrangement, and cannot be keyed by an open-drain driver. These
transmitters require a positive voltage to be sourced into their PTT inputs to
key; removing the voltage unkeys the transmitter. The current requirement can
be substantial.
A simple outboard circuit can be placed between the controller’s PTT output
and the transmitter’s PTT input to satisfy these requirements. (See schematic
on page B-3.)
• Connect a large PNP transistor so that its emitter goes to the transmitter’s
+12V supply, its collector goes to the transmitter’s PTT input, and its base
goes to the controller’s PTT output through a 2K resistor (important).
• Connect a 4.7K resistor across the PNP transistor’s base and emitter.
• If the transmitter draws little current through its PTT (500mA or less), you
can use a 2N2904.
• If the transmitter draws 1 Amp or less, use a TIP30.
• When using this outboard circuit, place DIP Switch #7 (part of S2) in the ON
(closed) position.
• You may then skip to the Repeater Receiver Audio section which follows.
Unless you have used the outboard circuit described above, you must
determine the sense of your transmitter’s PTT input.
• Does the transmitter key when the PTT input is driven low or high? (The
majority of transmitters key when the PTT input is driven low.)
• If the transmitter keys when the PTT input is driven low and unkeys when
the PTT input is opened, place DIP Switch #7 (part of S2) in the ON
(closed) position.
To Next Page
Loading...