Many times I have needed an additional status input on an already-installed remote control system after all the native options are used. Fortunately there are work-arounds.
I think everyone knows the ubiquitous ARC-16 - I'm sure there are thousands of them still in use. You can set up one of the analog inputs to mimic a status input easily enough (of course that means you have an analog input to work with). See Figure 1. Build a small voltage divider that has its low side connected across the available analog input. The top of the voltage divider has to come from a dc source - a wall wart will do. The dc across the analog input can't exceed 5.0V, so you'll chose your resistors based on that and the source dc. In other words, your resistor ratio can't exceed the voltage ratio:
If Source dc/5Vdc = X, then top resistor/bottom resistor > X.
This will keep you from exceeding the 5Vdc input limit on the ARC-16. Once you have this set-up connected, scale that channel's input calibration to read something convenient, like 500 or 1,000.
Clearly now when you short this lower resistor, the analog input will read zero. Take whatever device in your transmitter room that you intended to connect to a status input previously, and connect that across the lower resistor in the voltage divider. (Remember that when you do this, the wall-wart or other dc source will see the upper resistor as the load. Make sure the resistor can handle the power. In practice it's a good idea to use high values anyway (greater than 1kΩ)). A perfect example of this would be a squelch contact from a microwave or RPU receiver, or perhaps a magnet switch on a door.
The last step is to just set the lower limit on that analog channel to set off an alarm when its input reads zero, corresponding to the closed relay contact.
More than one
What if you need multiple status inputs? You can get around that requirement as well. Just like before, you'll need one spare analog input to work with. See Figure 2. This time we have the same normally open (N.O.) relay contact we had in Figure 1, which corresponds to Status A; we have a N.C. relay contact with a series resistor that corresponds to status B; and we have an additional N.O. contact with a series resistor, corresponding to Status C. Let R1 = 2.7kΩ, and let R2 = R3 = 1kΩ and let R4 = 470Ω. Various conditions will provide these results for analog voltage:
| Condition || Analog Voltage || Alarm Output |
|Status A||zero||low limit|
|Status B||3.24||high limit|
Set the low limit alarm to go off anytime the voltage goes below 0.99V; set the high limit alarm to go off any time the voltage reaches 3.24V (or somewhat lower, considering normal line voltage fluctuations in your source voltage). And there you go: one analog input can now distinguish five separate conditions, plus provide two alarm outputs. As the station's engineer, you'll map what all the conditions mean. You'll read these from the remote control or rely on it to call you.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.