Cheap low voltage alarm adjustable up to 36volts

[kfong]

It occurred to me from my radio control hobby that a low voltage alarm using a TL431 chip can be used up to 36 volts. This would be a very cheap indicator, either as backup alarm or on a setup such as a push scooter where simplicity is best.

The design is from Jimsky on rcgroups http://www.rcgroups.com/forums/showthread.php?t=221018&highlight=tl431

I created a board and recently needed an alarm for 4 A123's. I’ve been finding the A123 great as replacement batteries for many of my cordless stuff, but without knowing if the batteries need to be charged. I run the risk of damaging them. I had to come up with this board. It just sounds an alarm when the voltage hits a preset level. I replaced the 6.65K resistor with a pot so I can manually set the voltage to any set point I desire. The device works up to 37volts and any voltage up to 36 volts can be set with very good precision.

Here is a picture of the boards. I have some bare boards for $3 if interested. I designed the board to take different size buzzers. If the buzzer is too big, you just solder it on the backside. I needed it small to fit in planes and in cordless equipment.

 

[bzhwindtalker]

pretty cool!

 

[regmeister]

deleted

 

[kfong]

Yes, the boards are available. I have not tested it beyond 20volts. Waiting on some more TL431's before I do that and a buzzer that will go up to 36volts. It works great for the RC stuff and cordless tools. The components can go up to 36volts so I don't see any problems.
Part that does all the work http://focus.ti.com/lit/ds/symlink/tl431.pdf

The partslist
6.65K or a 20K Pot to make the setpoint variable. Note: 6.65K is the value set for 3 cell Li-Po packs. You will need to change this to whatever value needed for your app.
2.59K
1.5K
150 ohm
TL431
2N7000 or bs170(put in backwards, pinouts are reversed)
Buzzer that works with the voltage you will be running at. Make sure it's not a peizo speaker. They look the same but won't work.

As you can see, very low parts count and all fairly low cost.

It's a great way to use the A123's or any other type of batteries for other projects without having expensive monitoring stuff. I have some old power tools converted over to A123's and use this board as a way to tell me when the voltage is too low to use.

 

[regmeister]

deleted