Sorry for slow reply... busy weekend.
I don't think I've got it backwards, but nothing would surprise me
The transmitter comes with a Type 23A 12v battery in it, or at least it says 12v on it so I assume it's 12v. You can see it in attached pic "Front".
The receiver comes with 2 x 1.5v batteries, size AA. They go in the white box with red and blue wires shown towards the bottom of attached pic "Bell Back".

 

Hah! I feel stupid, my eyes were playing tricks on me. Can't believe I didn't see the giant 12V marking on that battery. Oh well, sorry.

As for using the 3.7V supply for the receiver, I honestly don't know. It would definitely be ok for the components that I fully understand, but I'm not able to wrap my head around the entire receiving circuit, so I can't say for sure whether or not it would respond differently at a slightly higher voltage. My gut feeling is that it would probably be ok, but I can't say that with any confidence.

Assuming the rest of the receiver is ok with 3.7V, I can certainly make our additions tolerant of at least a 3.0-3.7V range.

 

Not half as stupid as I feel when I look at your circuit diagram ! ! !
I tried connecting my 3.7v/355mA PSU (from an old Nokia mobile phone) to the receiver instead of the 2 x 1.5v batteries. Nothing happened. No ding-dongs, even when I changed the polarity. I hurriedly put back the batteries, hoping that I hadn't scrambled its brains, and all was well again, thank goodness.
Any idea why it wouldn't work with the PSU ? Does the mA number make any difference ?

 

Just a thought...
The 3.7v "PSU" that I tried is actually a mobile 'phone charger.
Is there a difference between a PSU and a charger, which would account for it not powering the receiver ?

 

Hmmm... would've expected it to work.

mA rating would only be a factor if the circuit needs to draw more current than that, which would surprise me.

It's possible that some part of the circuit really is just that finicky, maybe a comparator that will never trip because of the higher voltage, or something along those lines.

It's also possible that the charger is unregulated. Wall wart supplies often deliver their rated voltage only at the specified current, providing lower voltage at higher current and vice versa. The output of that charger might be a lot more than 3.7V with low current demands on it.

Once you've got your meter, you can measure the voltage out of the charger and determine whether or not it's a regulated output.

 

So, put a diode in series with the power supply/adapter to drop ~0.7v if the output is indeed 3.7v. Another test for when the multimeter arrives. Also, the meter can verify polarity.

No?

 

Multimeter now here. I'll look at it after dinner !

 

Multimeter seems to be working OK. Pic attached. I need to be told what section to set it to (e.g. DCV, DCA, etc), and what value within the section. And where to stick the red and black spikes ! ! ! ! !

 

Excellent! The probes are connected properly for most testing (would have to move red probe for most amperage tests, but we're not doing any of that, so no worries!) Our tests will all be DC voltage, probably between 3-5 volts, so "DCV 20" would be the range you want.

Here are the three tests I'd like to start with:
1) one probe on each of the two battery connections on the back of the receiver board
2) one probe each on pins 1 and 2 (top left and 2nd down on left) of melody chip
3) one probe each on the two connections of your 3.7V adapter with it plugged into the wall, but not connected to the receiver

If you let me know what each of those three voltage readings is, I'll do the last of the calculations and get you some resistor values for the diagram I posted earlier.

Also, never did get a good pic of the other side of the board. What's in between the battery connections and the speaker outputs? Is that a DC input jack? If there's a jack bridging those 4 solder joints in the top middle section there, then that would eliminate any uncertainty I have about the battery voltage being the exact same voltage that the melody chip sees.

 

OK, here's a better pic ("Receiver Front") on which I've noted what things are, or what I think they are.

I'll do the multimeter tests later.

 

Readings as follows :

(1) 2.90v
(2) 2.90v ( assuming the melody chip is the one with 4 solders on each side )
(3) 8.90v ( strange when it's a 3.7v charger ! maybe cos it's not drawing any amps ? )

djs fantasi : how do I use the multimeter to verify polarity ? If for example I have a PSU whose two outgoing wires are the same colour, and I touch the two multimeter probes to the ends of the two wires and it gives a negative reading (e.g. -8.90v), how can I tell which is the negative wire ?

I should have paid more attention in physics lessons at school

 

The red meter probe is meant to be positive, so:

If the meter gives a positive reading, the wire the red probe is touching is higher than the one the black probe is touching.

If meter reads negative, black probe is touching higher voltage.

 

Phone charger is almost certainly unregulated, would only deliver proper voltage with higher amp draw than the doorbell circuit pulls. Might be possible to use an ldo regulator to bring it down since the raw, unloaded voltage is so high, but I'm not sure about that.

Probably easier to get regulated supply to begin with, as close as possible to 3.0V and maybe add a diode as suggested earlier to drop a few tenths if the supply you get is a little over.

 

Thank you, I'll write that down on a bit of paper and stick it to the back of the multimeter !

 

Phone charger is almost certainly unregulated, would only deliver proper voltage with higher amp draw than the doorbell circuit pulls. Might be possible to use an ldo regulator to bring it down since the raw, unloaded voltage is so high, but I'm not sure about that.

Probably easier to get regulated supply to begin with, as close as possible to 3.0V and maybe add a diode as suggested earlier to drop a few tenths if the supply you get is a little over.

I've ordered a regulated one that you can switch from 3v up to 12v.

 

Thanks for the pic too. Everything makes more sense now!

 

Sweet! Good news on the power supply. Sounds like it should do the job.

 

All right! I think I've got it all worked out. Only thing I might worry about is whether the R1 resistor value might need to change, depending on the nature of the trigger pulse the existing receiver generates.

So here is the updated schematic with all the appropriate component values:

I would highly recommend bread-boarding this circuit before soldering anything, to make sure that the triggering is reliable, that the timing seems right, and to verify that everything has gone together properly.

Some of the voltage ratings, drops, and thresholds get a little tight on this design, so be careful if you make any substitutions on the 555(IC1) or the SCR(T1) in particular. The 555 I did my calculations on is the LMC555, and I also found that the ICM7555 should work. Non-CMOS 555s won't run on the low supply voltage, and the Radio Shack 555 listed online has a maximum reset voltage rating that might not work. As for the SCR, the key specs are holding current <=5mA and on-state voltage <=1.7V.

The switch in the circuit is drawn up as a normally-closed momentary switch, which can be hard to find at some brick and mortar electronics stores. You could also use a rocker switch - you'd just have to remember to flip it back to on right after each time you turned it off so that it would be ready for the next input.

 

When you've got this circuit done, it can be tested independently of the doorbell/receiver setup by just supplying it the 3 volts. If you momentarily touch the scr gate input line to the 3 volt line, that should start the timer and the LED should start blinking every 6-7 seconds (roughly 2 on, 4 off.) At any time, pushing the momentary switch will stop the timer, which will remain idle until the next time there's a voltage pulse on the scr gate input.

 

Thank you very much, ebeowulf17, that looks fantastic. I have a horrible feeling that it might be beyond my ability to get the right components and set them up on a breadboard, let alone fix the circuit to the doorbell receiver ! ! !

Couple of questions for starters :

1. On the left of your schematic there are three "wires" which go to "VCC_3.0VDC" and "GROUND" and "SCR-GATE-IN From doorbell receiver". I'm guessing that "VCC_3.0VDC" is the battery positive and that "GROUND" is the battery negative , but what is the "SCR-GATE-IN From doorbell receiver" ? Whereabouts on the doorbell receiver would I attach this ?

2. On the right side of the schematic there are two "wires", one of which goes "Out to melody chip 555-SQUARE" but the other isn't labelled, so where does that go ?

3. In the case of "Out to melody chip 555-SQUARE", for example, will I have to solder a wire to one of the 8 legs of the melody chip ? The legs are tiny, and the gaps between them tinier still, so I'm not at all sure that I'm capable of doing the soldering !

Bob