Advice on replacing or modifying analog buzzer

Thread Starter

Circuits123

Joined Dec 7, 2012
64
I'm modifying an old 1960s darkroom timer for use as a kitchen timer. It works fine but the buzzer could wake the dead, plus once it goes off the user must manually switch it off. So I'd like to either modify it so it sounds only briefly or replace it with something else. I'd prefer to keep the modifications analog. I've attached a photo. The red circle on the left is the on/off switch. The other is the buzzer. It runs on US household current.IMG_1847.jpg
 

djsfantasi

Joined Apr 11, 2010
5,693
Looks like everything is run from line voltage. So, I looked up a one shot time delay relay that runs on 120VAC.

I’m not recommending this device, a Littlefuse HRDS421, but it looks like it may suit your purposes. I’ve provided a link to its datasheet I’m letting you decide if it’s what you need.

You have to cut the wire that actuated the buzzer and wire it to the signal line of the relay. Then from the cut end going to the buzzer, wire it to the NO relay contact. Supply power to the relay and to the Common relay switch contact.
 

Thread Starter

Circuits123

Joined Dec 7, 2012
64
A time delay relay? That sounds just right. The one you linked to is pretty expensive but I found cheaper ones on ebay. Aside from being 120VAC, are there any other specs that you think are important for me to look out for?
 

djsfantasi

Joined Apr 11, 2010
5,693
There are two ratings you need. One, the coil operating voltage is 120VAC. The second is that the relay must be able to switch 120VAC and it needs to be able to handle the current draw of your buzzer.

And you want a one shot relay...

These requirements are probably not difficult to meet.
 

Thread Starter

Circuits123

Joined Dec 7, 2012
64
I've seen a number of relays for sale that have a schematic along these lines:
upload_2019-7-22_13-56-31.png

Which pins would I use in my case? - and thanks again
 

djsfantasi

Joined Apr 11, 2010
5,693
I've seen a number of relays for sale that have a schematic along these lines:
View attachment 182216

Which pins would I use in my case? - and thanks again
This isn’t a “one-shot” relay. Thus once activated, it will stay on. What it does if deactivated, it will take 30 seconds to turn off. It may be able to create a circuit to do what you want with the addition of another 120VAC relay with 120VAC contacts.

The alarm signal is wired to one side of the first relay; the second side is wired to a common point of 120VAC.

The delay relay coil (pin 2) is wired to the NC contact of the first relay; the second side is wired to a common point of 120 VAC.

The corresponding common pin of the first relay is connected to one side of 120VAC.

The NO pin of the first relay is wired to the common pin (pin 1) of the delay relay. Then the buzzer is wired to the NC contacts (pins 1 & 4) of the delay relay. The other buzzer connection is wired to s common 120 VAC point.

A schematic would really help, but I can’t make one right now.

This complexity can be avoided by the right type of relay. You need a 120VAC One-Shot relay...
 

Thread Starter

Circuits123

Joined Dec 7, 2012
64
You have to cut the wire that actuated the buzzer and wire it to the signal line of the relay. Then from the cut end going to the buzzer, wire it to the NO relay contact. Supply power to the relay and to the Common relay switch contact.
So the relay arrived in the mail recently and now it the weekend and I'm trying to figure out how to connect it. This is the one I got. It looks like pins 2 and 10 are the positive and negative terminals for the power supply. I guess it's not polarized. It's a DPDT relay so I guess it has two Normally Open contacts at pins 3 and 9? And that would make pins 1 and 11 the common contacts? But I can't figure out which is the supply. And since the relay needs its own power supply I guess I have to add a couple extra wires for that? Thanks for your patience.
 

djsfantasi

Joined Apr 11, 2010
5,693
Pins 2 and 10 are connected to the switched supply.

You’re correct in your analysis of pins 1 & 3, as well as 9 & 11. Pins 4 & 8 are NC contacts. You won’t need them.
One of the power lines of your buzzer is cut and one side is wired to pin 1, while the other side is connected to pin 3.

Note you want to use mode 2. This gives you one-shot when power is applied to pins 2 & 20. So, you need to connect pins 5 & 6 per the datasheet.
 

Thread Starter

Circuits123

Joined Dec 7, 2012
64
I'm getting close. But there's one side effect that I'd like to know if I can avoid - the buzzer sounds a little strange now, it seems to studder. I wonder if that's because the timed relay is sapping some of the current or voltage? Here's a photo of how I've connected it:

IMG_1847.jpg
The other end of the white wire #1 is disconnected from the upper screw (not where the number label is) and reattached to pins 2 and 3. The red wire #2 is also disconnected from the same screw (that's an outlet that I don't need) and connected to pins 1 and 10.

So now it works. When the timer hits zero, the red light on the relay comes on and the buzzer goes off for whatever time is set on the relay. But the buzzer sounds funny. Do you think there's anything that can be done about that? Maybe another place to connect pins 2 and 10?
 
Last edited:

djsfantasi

Joined Apr 11, 2010
5,693
Not sure why you think the buzzer has less voltage/current. If wired correctly, that won’t happen.

Can you sketch out or draw a schematic?
 

Thread Starter

Circuits123

Joined Dec 7, 2012
64
I'm just speculating that the change in buzzer sound could be the reduced voltage/current. It sounds like it has less power than before. Here's a very rough schematic. A couple explanations: (1) the focus/timer switch is a toggle switch with 2 positions (same as the on/off switch) but on the back it has six contacts; (2) the thing on the bottom right is the time delay relay.IMG_1962.JPG
 
Top