CDM1250 as a station alerting base

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OldDawg
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Posts: 41
Joined: Tue Apr 05, 2016 1:43 pm

CDM1250 as a station alerting base

Post by OldDawg »

I donno what I might be doing wrong here........

I have 3 radios here....CDM1250's......all programmed the same.....trying to get an external alarm output across pins 4 & 7. Pin 4 programmed to go low.

Programmed for select call.....no switch. Programmed for call alert......no switch. constant 12 vdc on pin 4 to ground with power applied to the radio, even if powered off.

One of these radios is new old inventory. Can't have 3 bad radios.......or maybe I do....?
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kb4mdz
Posts: 282
Joined: Tue Oct 02, 2001 4:00 pm
What radios do you own?: Too many for the time I have.

Re: CDM1250 as a station alerting base

Post by kb4mdz »

The Devil, as they say, is in the details...
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jackhackett
Posts: 1505
Joined: Tue Jun 10, 2003 8:52 am

Re: CDM1250 as a station alerting base

Post by jackhackett »

The pin 4 circuit is an NPN Darlington transistor, emitter grounded, collector pulled up through a 4.7K resistor to FLT_A+ (filtered A+), pin 4 connects to the collector.

Because of the pullup you will measure 12V there even if the radio is off, when activated as active low it should go down close to ground, rating is max low voltage 1.7V with a 200mA load.

You generally use that pin to drive an automotive relay, pin 4 goes to 85 on the relay, 86 goes to +12V.

If pin 4 doesn't go low when activated either it's not actually getting activated, you're measuring the wrong pin, or the transistor is fried.

You can test it by programming it for external alarm active high. This will make the output normally low so with nothing connected to pin 4 you should read around 0.6V and it will go to 12V when activated.
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