Mrs Rog likes a bit of music when she is working, in the corner of her office a battery operated under shelf radio alarm clock had been fitted. It works fine...except that when the phone rings she has to cross the room to turn the radio off before picking up the call... then cross the room again to turn it back on, not an ideal situation at all.
Step 1: Research and Development
The first attempt was an interrupted power supply, I tried 2 methods, a plug in mains supply or an insert to cut the battery continuity, both failed due to this being a radio alarm clock. When the power is removed then reapplied the alarm sounds!
The only other way that could be an easy fix was to remotely cut out the loud speaker and replace it with a dummy load.
In the remote control department of the workshop I had a 2 channel momentary change over fob and board, which means the relay operates only while the button is being pressed, not entirely suitable but with an additional DPDT relay wired to latch in the on position the design was sorted.
I expected to be building this into a separate project box with a flying lead to connect it up.
Remote fob and relay unit
DPDT 9v relay
Battery connector clip
Resistor to match impedance of loud speaker
Odds and ends of insulated wire
Double sided tape
Foam rubber packing
Step 2: The Build 01
When I opened up the radio I discovered wide open spaces where all of my bits could be mounted with room to spare, result!
It was immediately obvious that if I could mount the battery under the slide that is used to attach it to a shelf the radio would not need to be disassembled when the battery needs replacing. so out with the trusty Dremel with a saw disc mounted.
The hole was roughed out and then tidied up with a bit of filing.
Step 3: The Build 02
I cut a piece of thermo-formable plastic out and using a vice and a hot air gun bent it to shape to contain the PP3 battery.
Some foam rubber was then attached with double sided tape to keep the battery still.
The holder was then super glued in place.
Step 4: The Build 03
I soldered a piece of Vero-board onto the latching relay then stuck it into a space in the radio and proceeded to solder the wires in place to swap the speaker for a dummy load (resistors to the same value as the speaker) when the relay operates.
I then attached the necessary wires to the remote unit and got ready to double sided tape it in place within the radio.
Step 5: The Final Touches
The remote unit was stuck down and the final connections to the battery housing were made.
The radio was reassembled, batteries fitted and it was time try it out!
With the radio on button "A" was pressed and the radio went silent, as the relay has been wired to latch the remote button can be released.
A press on button "B" cuts the supply to the relay which turns off and the radio comes back on, Success!!!!
Step 6: Soak Testing and Resultant Modification
A job is never finished until it is fully tested, after writing this instructable last night I returned to the workshop.
I wired the remote up to my bench power supply and found the lowest voltage that would still operate it successfully, 8.0V was the spot at where the radio would turn off but would not turn back on. There are a couple of reasons that made this result not unexpected, the first is that the remote is designed for 12v operation, I just knew from experience that 9v would do the job as long as the battery was not too far discharged.
The second part was that as the battery is used to hold the latching relay on its voltage is pulled down enough for button "B" to no longer operate, meaning the radio would not come back on.
So if the phone rings the radio can be muted but at the end of the call it can not be un-muted if the battery is low on volts. A walk across the office to disconnect the battery would be the quick fix until a new battery is fitted. but an isolator switch would be better!
There is room under the shelf slide for a roller operated microswitch.
Step 7: That's Better
Once the correct position was worked out a pair of holes were drilled to allow the wires through to the inside of the radio. The switch was adjusted and screwed in place then the remote was wired up again.
Mrs Rog can still mute the radio with a pretty flat battery so the phone call can be taken quickly.
To turn back on she just has to slide the radio forward an inch or so to reset... she could while she is there replace the battery, but that is up to her.
Project signed off and delivered!