Boy howdy-this is a neat site!

Never taken formal electronics classes-but am planning to late in my career. I have taken on a project-unique as it may be, it is probably a snap for all you seasoned ones.

I am looking for an out of the box solution if available. I am working on a restoration project for a historic site here in CA. The application is a timer that starts counting down once energized, and counts down one minute-and at the 58, 59 and 60 second mark, provides a -48vdc pulse to ring a bell three times-then the cycle starts again until the system is de energized.

The timer should be a solid state unit powered by 110ac input, and the output will be -48vdc to drive the bells.

In what I have read so far, the 555 timer appears to be a workhorse timer. I have also seen timer kits on line (pre assembled and kits). The tricky part is the output for the -48vdc bells. I would be quite estatic if the board could assist me in resolving this project.

Thank you for your time ~~!

DavidB

 

I don't think 555 can be a very reliable option. I have not worked much with the 555 but the little amount of work that I have done with it, It was not very reliable for my particular application.
I would use a microcontroller and create as accurate as possible delay, i would also use the microcontroller to switch on and off the supply to the -48vdc bell using a relay (maybe) or why not just a transistor.

 

I doubt that you are going to find an OEM solution for a 1 minute counter that provides a pulse on each of the last three seconds (also, see below regarding bell ring time).

To build one:
First you would need to make a -48V power supply using a 120VAC to dual 34VAC (or 64V CT) power transformer and a full-wave rectifier/filter, which would go to the switch contacts of an appropriately sized relay which will turn power on/off to the bell system (you don't state the amount of current needed for the bell system).

Then, you also add a diode that taps off the signal from the secondary of the xfmr that will give you a positive voltage at 30Hz (half-wave), and adjust it for TTL levels of approx 0V to +5V. You run this into a schmitt trigger inverter (74HCT14) and use this output for a clock. This clock is then divided (as /3 /5 /8) to take the 30Hz signal down to 0.25Hz, and apply this 0.25Hz to a 16-bit (0-15) counter, you "OR" together counts 13, 14, and 15 and run this output into a transistor that can drive the relay coil. Count 15 also resets the counter to start over (may need some delay, string together left-over HCT14 inverters). This is not a one-minute counter, nor does it do exactly as you described, it will take longer because there needs to be time (~4 sec.) between bell pulses in order for each bell tone to fade so the ear can distinguish the distinct bell tones (diiiiinng, diiiiinng, diiiiinng) and this also allows for binary counting boundaries. If you were to produce a pulse on seconds 58, 59 ,and 60 as you described, I think that essentially all you would hear is one ding 3 seconds long. Or, perhaps two very fast ones and a then a third that fades (ding,ding, diiiiinng -- which may not sound right).

There are other ways of doing the above, and ORing the counts may not be easy with glue logic (as opposed to using state-machines in a PLD). For instance, using BCD counters combined with binary counters could make it easier to build. One could also raise the clock frequency by dividing it by less, and then use a larger counter for better granularity in the counting and timing.

 

Am I missing something here? Given two terminals (call 'em T1 & T2), with a 48VDC differential between them, if I place the negative (black) probe of my multimeter on T1, and the positive (red) probe on T2, and I get a reading of +48VDC, and I then switch the two probes, I should get a reading of -48VDC.

--Rich