How to make a constant buzzer beep non symmetrically? - Automotive Low Battery Alarm (Solved)

Thread Starter

mvs90

Joined Mar 20, 2012
24
Hi there,

I'm interested in modifying an off the shelf, automotive low battery voltage alarm. The devices inputs are Battery Voltage and Ground, and there is a single pot to modify the voltage at which the alarm triggers. I imagine that in essence it's little more than a comparator.

The issue is that once below the threshold voltage (say 10v for a normal automotive battery), the buzzer buzzes continously. That creates three issues. First, it drains a low battery even faster if you don't notice, possibly damaging it. Second it's a bit ridiculous, as well as annoying, to have it beeping continuously even after you've already connected the vehicle to a charger. And third, some chargers will not charge your car battery if they detect the continous drain which is a beeper buzzing, since it detects it as the battery not charging properly and shuts down.

Oddly enough, I haven't found any commercial low battery alarm that does this. At least not cheap and compact. The low battery alarm is less than 5 dollars.

All I want is something to put at the output of the device, before the buzzer, which would make the buzzer beep for a fraction of a second, stop for 10-30 seconds, beep for a fraction of a second, and so on. I'd like to use a 555 timer should it be suitable, but I've only seen how to easily make it "beep" symmetrically, as in 10 seconds on, 10 seconds off, and so on. That wouldn't be worth the effort for me.

The voltages would be less than 15v, and low currents. also don't know if the output voltage is constant until the battery drains, or if it decreases proportionally. I can look up both if needed, but don't know a priori.

I would be using either one of the following two circuits (I already have both).

First option:
s-l500 (1).jpg

Second option:
s-l1600.jpg


Is there any easy solution for this? Or better still if someone can point me to some post from someone who had the same issue? I didn't quite find it.
 

Bernard

Joined Aug 7, 2008
5,783
Seems to me that as 555 should work. 10 uF timing cap., 20 k Vcc to pin7, 1 M & diode in parallel from pin 7 to pin6, anode to 7, 6 & 2 tied together. Buzzer from pin 3 to ground if drawing les than 200 mA.
 

Thread Starter

mvs90

Joined Mar 20, 2012
24
Seems to me that as 555 should work.
Thanks for the quick response!

Could you point to some "tutorial" type page (or tell me what to search for) where I can see how and why it works, and how to decide on the values? Also it would be great if both the on and the off times were variable. Would that configuration permit?

Now that I think about it...are you talking about an astable configuration of a 555 timer? When I looked at this some time ago I think I ended up finding that it couldn't be used for a duty cycle of less than 50%. And this case it would probably be a duty cycle in the 0.003% or less range.

I'm looking for a result like the low battery alarm of a smoke detector.
 
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GopherT

Joined Nov 23, 2012
8,012
You could use a window comparator (two comparitors) that allows the buzzer to go off only when between 9 and 10 volts, for example and turn off again below 9. That would require an off-switch for the buzzer if you didn't want it buzzing while charging (or to stop buzzer until you can connect the charger).
 

Thread Starter

mvs90

Joined Mar 20, 2012
24
That would require an off-switch for the buzzer if you didn't want it buzzing while charging
The alarm would be buried inside the vehicle, and having a switch on an automatic alarm kind of defeats the purpose. If I miss the buzzer between 9v and 10v for whatever reason, then I'd never notice that the battery is low so that doesn't sound like a good solution.

I'm currently breadboarding this design. Let's see what happens in a few mins.

Update: output voltage from low battery alarm to buzzer varies with the battery voltage. So the voltage originally outputted to the buzzer varies between around 0v and battery voltage (10v in practice, 15v in possibility).
 
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BR-549

Joined Sep 22, 2013
4,938
If I were gonna monitor the voltage of a battery........I would test it periodically with a current draw. This current draw would be timed. Let's say we turn the current on every 30 seconds for one half second. A timed switch. The voltage drop of this current can be compared to a reference. So, when appropriate, the comparator may turn on a beeper. For one half second. Of course you can change your timing with the switch.
The comparator, the reference and beeper only need power during test current time.

There are many ways to do this. Just a strategy suggestion.
 

Thread Starter

mvs90

Joined Mar 20, 2012
24
If I were gonna monitor the voltage of a battery........I would test it periodically with a current draw.
Thanks for the response. Out of curiosity, what would be the benefit? Given the nature of this project (quick and dirty estimation of battery condition) I think simply sampling of the voltage is sufficient. No doubt loading the battery makes a better representation of the actual battery state. If I wanted to know with relative precision the percentage of charge the battery has left, I think your method is the way to go, but for a simple pass/fail test of battery condition, I think that's overkill. But maybe I'm not seeing something. However if it has a sub mA standby current (unlike my original off the shelf solution) than it just became more attractive. But even then I'd probably still think sampling voltage instead of current is still preferable (for simplicity).

Okay, so I finished the project. I ended up going with this circuit, modifying the timing values. I think it's what Bernard was referring to before. I'll have to take a look at exactly how it works later, but I did get it working.

F9YDLNDH0KL4OAJ.MEDIUM.jpg

Finished circuit:
2017-02-06 23.14.37.jpg

But...sadly, after finishing, I decided to check current consumption. I expected sub mA when voltage is good and the circuit has nothing to do, close to that when the battery is low but the buzzer isn't buzzing, and whatever when the buzzer was.

I actually found 10mA when the battery is good, and around 6mA when it was low. So the Low Battery Alarm actually is partly responsible in draining the battery to begin with... This is for a motorcycle, where the battery has to be charged if it isn't used at least once a week (GPS Tracker Alarm + other random electronics), so 10mA of constant battery drain doesn't really attract me at all. I don't think I'm going to install it.

Oh well. Experience gained.

Eventually I'll probably go with a small microcontroller which goes to sleep and simply wakes up every so often to check, and then back to sleep. Implemented right I think it's reasonable to expect a sub-mA drain from that.
 
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BR-549

Joined Sep 22, 2013
4,938
The current draw only needs to be a few milliamps every 30 seconds or so. About one second out of every minute. You can adjust this timing.
They have very low power timer chips now.
 

Dodgydave

Joined Jun 22, 2012
9,938
I would modify the original pcb with the Lm358, can you draw the circuit out if not, post a pic of the trackside and mark the components out, that way it will be easier...
 

AnalogKid

Joined Aug 1, 2013
9,178
The circuit in post #7 is a good approach, but it has issues. For the standard (non-CMOS) 555, both the 4.7 M resistor and the 100 uF capacitor are a bit large. You are into the area where the leakage current of the capacitor is a significant fraction of or even exceeds the charging current through the resistor, seriously affecting the timing value and its stability. Also, the off time is 8.6 minutes, not 10-30 seconds.

Another thing about the bipolar 555 is that it is hungry. The CMOS part has a much lower static current.

There is another approach to this that uses a CD4060 oscillator/divider as the timing element, plus a 2N7002 transistor to drive the buzzer. Very low operating current and much better timing performance, but it adds two small diodes and one resistor to the BOM.

ak
 

AnalogKid

Joined Aug 1, 2013
9,178
One of the inefficiencies in a 555 circuit is that the charge built up in the timing capacitor is dumped to GND rather than used. Year ago, National Semiconductor made the LM3909, a super-low power LED blinker. It charged up a timing capacitor, then dumped the capacitor into the LED. Very low operating current and no current spike when the LED flashed. Now that I've remembered this, I'm going to have to work out a low cost way to replicate that for a buzzer.

ak
 

eetech00

Joined Jun 8, 2013
2,318
Hi

Try an TLC555...it is a CMOS version of the 555 and draws very little current (~350uA). There is also an automotive version of this chip...TLC555-Q1
 

AnalogKid

Joined Aug 1, 2013
9,178
I've never used a CMOS opamp, but my circuit needs very low static current. Any recommendations? Single amp, 18 V Vdd, rail-to-rail in and out, don't care about bandwidth, offset, etc.

ak
 

Thread Starter

mvs90

Joined Mar 20, 2012
24
Why not just hook the battery up to a trickle charger when it's not being used?
Because no one feels like doing that on a regular basis, nor having to unplug before. With Florida weather you never know if it'll be a day or a few weeks without riding. 90% of the time the batteries never get's discharged. But it's that 2 or 3 times a year that it does that kill batteries. Deep discharge is the biggest killer of most lead-acid batteries.

Besides, if we all applied that mentality, half of the projects on this site could be answered along the same lines. Not really much charm to that.

There are lower current draw battery gauges available ... might require some assembly though. What is the A-Hr capacity of your motorcycle battery?
This gauge goes up to 29 A-Hr ... maybe not enough for your application?
I already have a negligible current draw battery charge indicator. The problem with that is that none seem to integrate an alarm or be small and cheap enough for a motorcycle. To make matters worse, the battery is lithium, so voltage readings typically show good and then quickly show bad, with not much of a transition.

I wish! The battery is 9 A-Hr (hence my concern with parasitic currents). My last Lithium battery was even worse at 6 aHr. But same issue. I only need a battery alarm since I already have a monitor, it's just that it's easy to miss since it's unrealistic to expect to foresee a low battery.

The circuit in post #7 is a good approach, but it has issues
The current draw issue wasn't due to what I added, it was due to the off the shelf low voltage alarm that I modded. I'm fine with 6-10mA of draw while the alarm is sounding every 45 seconds since presumably I'll notice it within hours. It's the upstream circuit taking 10mA while the whole thing does absolutely nothing that is the deal breaker. So the 555 doesn't really play any role in what bothers me, that worked fine.

Timing is completely irrelevant in this case. It could be as sloppy as it wants and make no difference since it only has to catch one's attention. This was a cheap fast and simple project after all.

Yeah, I know. I modified the values using a 3k ohm resistor and a 2M ohm resistor for a second long beep every 45 seconds. That seemed like the best interval after playing with it.

Modding the original low voltage alarm further doesn't seem like it's worth the effort. If I take this project up again in the future, the option that most attracts me would probably be to implement it via a ATTiny85 or something like that. It would probably offer the most flexibility for the least amount of space for a casual hobbyist like me.

Thanks for all the input from everyone. I think this post can officially be considered solved.
 
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