Relay on alarm clock board does not close

[lti]

I have a Chaney Electronics/Electronic Goldmine C6952 alarm clock/timer, which has an Omron G6HU-2 relay that can be used to switch power on or off to another device when the alarm goes off. The microcontroller does not energize the coil long enough to latch the relay contacts closed. Is there any way to keep the coil energized for a longer period of time? I can make the relay latch and reset using a battery pack. The board uses an ATMEGA48-20PU microcontroller.

[Longbow]

Re: Relay on alarm clock board does not close

If this is a consumer bedside alarm clock, it is unlikely that it contains a latching relay. I would expect a solid turn-on signal from the micro going to the base of a transistor, which would ground the low end of the relay. What is the output from the micro, a pulse or a continuous voltage?

[lti]

Re: Relay on alarm clock board does not close

I just saw your reply.

This is actually a clock kit. It does use a latching relay and the microcontroller sends a pulse to the relay, but the pulse is not long enough to make the relay latch on. Both of the pins connected to the coil are connected to the microcontroller.

[joshnz]

Re: Relay on alarm clock board does not close

Both of the pins connected to the coil are connected to the microcontroller.

That is odd must be a custom ic.
I would have figured that one of the pins be connected to + and the other to a transistor.

[lti]

Re: Relay on alarm clock board does not close

Like I said above, the IC is a preprogrammed ATMEGA48-20PU. Since it uses a latching relay, a pulse is sent across the coil to make the relay contacts close and a pulse is sent in the opposite polarity to make the contacts open.

[joshnz]

Re: Relay on alarm clock board does not close

ah I see now hmm no easy solutions come to mind.
Maybe a bipolar cap across the output but that may overload the mcu outputs.

[b700029]

Re: Relay on alarm clock board does not close

Like I said above, the IC is a preprogrammed ATMEGA48-20PU. Since it uses a latching relay, a pulse is sent across the coil to make the relay contacts close and a pulse is sent in the opposite polarity to make the contacts open.

Software solution: read the code from the MCU, disassemble it and find where it sends the pulse, and modify probably a delay constant to increase the duration. Flash the code back to it.

Hardware solution: H bridge, several diodes, and an R-C delay circuit.

[lti]

Re: Relay on alarm clock board does not close

I don't want to buy the programming tools right now. What circuit would I need to build for the hardware solution?

[b700029]

Re: Relay on alarm clock board does not close

You need nothing more than a few wires and a parallel port to program the AVR.

For the hardware solution, you need to make an H bridge (4 transistors) to be able to drive the relay with both polarities, use diodes connected to the MCU outputs to control the H bridge according to the polarity, and add RC delay circuit to "hold" the selected output for however long you need to.

[Longbow]

Re: Relay on alarm clock board does not close

It is very unlikely that the code in the micro is bad. Since the drive comes from the micro itself, be sure that your B+ is solid and within spec. A more productive approach would be to experiment with the relay itself to see how much current is required to operate it. Could the relay be defective, or the wrong part for this kit? What do the markings indicate? Second phase: latch the relay by hand and see if the micro can unlatch it. It would be very simple to take the latching signals from the micro and use them to operate a simple flip flop driving a standard relay. I've never seen a radio with an accessory outlet. Seems like a weird feature.

[lti]

Re: Relay on alarm clock board does not close

The markings match what the kit showed in the parts list. The microcontroller is able to unlatch the relay, and the relay can be latched using three dead AAA batteries. The entire board is powered by a 7805 that produces 4.90V. The kit came with a 78L05, but I didn't like how hot it ran. The regulator is soldered to the red and black wires on the corner of the board.

[b700029]

Re: Relay on alarm clock board does not close

Operate (set) time 3 ms max. (mean value: approx. 2.0 ms)
Release (reset) time 2 ms max. (mean value: approx. 1.0 ms)

The coil takes 20mA which should be OK with the MCU since it can supply up to 40mA.

[lti]

Re: Relay on alarm clock board does not close

Maybe the microcontroler is programmed incorrectly. I could see that happening. Chaney Electronics doesn't make many kits that work properly. If it was possible to extract the program from the microcontroller and modify it, the problem could be fixed.

[b700029]

Re: Relay on alarm clock board does not close

Yes, it's possible unless they locked it, which might not be the case.

The hardware is really simple, just a few wires to the appropriate pins.

Use software like AVRDUDE to read/write to it:
http://www.nongnu.org/avrdude/

Once you have the program, you'll need to load it in a disassembler to get readable code. Use the datasheets as reference to understanding the program. There will probably be a small delay loop that you'll need to change so it waits longer. Then reassemble and write it back to the MCU.

If this is your first time working with microcontrollers, you should start reading the datasheets/manuals. There is lots of information

[Longbow]

Re: Relay on alarm clock board does not close

4.9 volts coming from a 7805 means the regulator is faulty. Use a new regulator. Then put a diode in the grounded leg which will raise the output to 5.6 volts. That should solve your problem.

Interesting how much enthusiasm there is for reprogramming microprocessors, even for a clock radio. That's the spirit!

[lti]

Re: Relay on alarm clock board does not close

The regulator is new, and its datasheet shows that the minimum output voltage is 4.85V. Also, the ICs on the board are only rated for 5.25V. I might try reprogramming the microcontroller.

[lti]

Re: Relay on alarm clock board does not close

I want to start messing with this again. I need to figure out how to create the programmer and what software is needed to extract the program, disassemble it, and reprogram the microcontroller with the modified program.

One possible problem is that the TXD and RXD pins on the microcontroller are used for the DIP switch bank.

[japlytic]

Re: Relay on alarm clock board does not close

A number of pre-programmed microcontrollers are code protected, which means you cannot extract the code from them.

[b700029]

Re: Relay on alarm clock board does not close



^ datasheet for your MCU, with all the info you need (see the "memory programming" section.)

http://www.arduino.cc/en/Hacking/ParallelProgrammer
http://reprap.org/wiki/Burning_the_S...der_using_DAPA

^ simple programmer that plugs into the parallel port.

You'll need an AVR disassembler and assembler. I think you can find those on Atmel's site.

A number of pre-programmed microcontrollers are code protected, which means you cannot extract the code from them.

This is true, however you should at least be able to read the signature bytes/lock bits to check whether it's been protected. In the worst case with the AVRs you can still erase the entire chip and now you have a blank MCU to write your own program to.