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Author Topic: How get around momentary power switch?  (Read 5932 times)
KG6SII
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Posts: 27




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« on: November 11, 2011, 11:04:15 PM »

I want my mobile APRS set-up to beacon whenever accessory power (the ignition) is on, but my radio (FT-2900) has a pesky momentary power switch!  It does not have an auxiliary input to signal it to power on.

Anybody know of a work-around for this situation, to get the transceiver to power-on when it gets 12v power?

Thanks much,
KG6SII
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K7RBW
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Posts: 390




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« Reply #1 on: November 13, 2011, 05:44:42 AM »

On some radios, if you remove the power when the radio is on, it will come back on when the power is restored. I don't know if your radio does that, but it's worth a try. I have a Kenwood dual-band that acts that way. If it's on when the DC power is removed, it comes back on when the power is restored.
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AA4HA
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Posts: 1418




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« Reply #2 on: November 13, 2011, 12:17:17 PM »

How about an isolating diode on the 12 volts to the radio and a big (60,000 uFd) cap to carry the radio past the momentary loss of DC supply. The isolating diode would be to prevent the cap from keeping vehicle things from seeing power during the move from the key in the running position to aux and also to keep the radio from resetting when cold cranking the battery (the vehicle can see the battery voltage drop and may shut down momentarily.

The cap value of 60,000 is sort of arbitrary. If someone wanted to get egg-headed about it they could calculate the load by the radio and the drop from 13.5 VDC down to whatever voltage threshold that causes the radio to blip. You just need to remember that there is a 0.7 volt drop across the diode that may affect your power level.

The diode needs to be sized big enough to support the full running current of the radio as well. The cap has an additional benefit of eliminating any sort of alternator whine or RFI from the vehicle electronics.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
K7RBW
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Posts: 390




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« Reply #3 on: November 14, 2011, 07:50:57 AM »

I read the problem as being that you couldn't set the radio in the "on" position so that when power was applied, it would come on and when power was removed, it would go off. While I don't know that particular radio, most of the new mobile radios I've seen use a "soft" power switch that basically boots-up the radio's electronics like a computer. Touch-on/touch-off. Some have a memory that will return the radio to the previous state when power is applied such that if it was on when power was removed, it will come back on when power is restored. While I suppose others might just reset to the power-off state.

It's not like the "old days" when a radio had a hard power switch that actually fed the primary power to the [not-as-computerized] electronics in the radio.
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AF6OF
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Posts: 27




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« Reply #4 on: November 15, 2011, 10:23:39 AM »

If you can get into the internal switch contacts, you could build a simple "one shot" timer that would create a momentary open collector transistor switch to "push" the switch every time power comes on. It would be a lot less ugly than a solenoid. As far as a back-up capacitor, as long as your unit stays awake, the receiver will be drawing enormous amounts of power ( relatively), requiring capacitor values in Farads, not MicroFarads,  so a back up battery ( Gelcel, etc.) would be a better choice. This would keep your radio on all the time, which could be better achieved ( if that's what you really wanted) by connecting the radio directly to the car battery. Better start the car pretty often though! I drew from your question that your goal was to turn the radio on coincidentally with your ignition switch, and the best way to do that ( in my humble opinion) would be the method I suggested first. (Or you could train a small rodent to push the button when he sees the ignition lights come on, but well-trained rodents are in short supply these days)

73,

Allen AF6OF
VHS/Byonics
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AA4PB
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Posts: 12832




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« Reply #5 on: November 16, 2011, 08:07:26 AM »

I would run this test: Hold the momentary power button in while you apply external power and see if the radio turns on and works okay. If so, you can just place a jumper across the switch. Whether or not this works depends entirely on how the firmware was written to detect the button press.

You also want to test to ensure that the radio doesn't mind having the DC power removed without first turning it off with the momentary switch. Again, depending on the firmware, some radios may have issues with intermittent memory loss if you kill power while it is operating. That's especially true if there are any large filter caps that might cause the applied voltage to ramp down gradually in lieu of going to zero immediately. Some processors get confused if they are operated at a lower than normal voltage.
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KD5KFL
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Posts: 55




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« Reply #6 on: November 27, 2011, 05:09:16 PM »

old $25 swapmeet radios. Kenwood TR7400 is the best. set em up once, no pesky memory batteries, just dial in the right frequency and mount them out of the way.
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AF6OF
Member

Posts: 27




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« Reply #7 on: November 27, 2011, 09:08:44 PM »

Have you looked at our MT-RTG FA? A ten Watt stand-alone tracker in a a cigarette pak side case, no bulky old, power gobbling ham radio to eat power and take up space; http://www.byonics.com/mt-rtg This way you can keep your radio for voice!

Allen AF6OF
VHS/Byonics
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