Voltage Monitor for Mag?

[xul]

Yes. Hobbyking has really cheap stuff.

That seem less than the cost of parts???

 

[moderator007]

Usually you have to buy more than one thing to come out with the price of shipping.

 

[The_bad_Frag]

I want to order a few things from hobbyking anyway so shipping is not a problem.

@xul Hmmm I dont know how much the parts cost but I dont think they miscalculated it. If you buy a huge amount of parts you can maybe get the price that low... More important is how I can get it buzzing at 3,6V.:devil:

 

[xul]

More important is how I can get it buzzing at 3,6V.:devil:

Post its schematic or at least the numbers off the top of any ICs in it.

 

[DIWdiver]

I know I'm a bit late to the game, but just saw this thread and thought I'd put in my 2 cents.

The circuit referenced by Norm in post#2 (look at post #1 in that thread) should work fine over the entire range of battery voltage that might be expected. It would be easy and inexpensive to build, even for a noob.

In this circuit the '431 is operating like a comparator with internal voltage reference, so it's similar to the more complicated circuit posted later in that thread.

For your lower voltage, you want to use a single LED and a 1.25V version of the '431, and adjust the resistor values. I'll explain how to do that.

Here's how it works: when the reference pin (pin 1 in the circuit of post #1) is less than 1.25V, the cathode current will be low. The cathode current flows from the cathode (pin 3) to the anode (pin 2). When the cathode current is low, current will flow through R1 to the LED, lighting it up.
When the voltage at the ref pin is above 1.25V, the device will pull the cathode voltage down to 1.25V. This is not enough to light the LED.

Both the cathode current and the LED current are controlled by the series resistor (R1 in the schematic). You want to select R1 to limit the current to a safe level for both the LED and the 431. The maximum current for the '431 depends on which part you select, so be careful.

I like the TLVH431QLPR part for noob builds. It's in a package that's easy to work with, allows for a maximum current of 80 mA, and the whole project can be done for less than $1.00 in parts and can fit on a piece of perf board the size of your thumbnail. However, there are many other choices available if you know what you are looking for.

The minimum value of R1 is determined by the maximum LED current, or the maximum '431 current, whichever is less (probably the LED), or a lower current if you choose.

Lets say you have a red LED with 1.7V forward voltage and 20mA maximum current. The formula for R1 is R1 = (Vbatt-Vled)/current. Plugging in our numbers, we get (3.6V-1.7V)/0.020A = 95 ohms. This would be the minimum value, so we might select a value of 100 ohms. For a current of 5mA, we'd want 380 ohms. So you have a big range to play in, and there's no need for precision.

You will also need to select the value of R3 to get the cutoff voltage you want. The formula is R3 = 1.25*R2/(V-1.25), where V is the desired cutoff voltage. If you want 3.6V, then R3 = 1.25 * 4700 /(3.6-1.25) = 2500 ohms.

If you want a two-level indicator, just build two circuits and set the voltages different!

(More than) A few words on accuracy:
The TLVH431QLPR has an accuracy of 1.5%. If you select 1% resistors for R2 and R3, you'd expect an overall accuracy of about 2% (I won't go into how I got that). At 0.9V/cell, this gives you a range of 0.88 - 0.92V/cell where the LED comes on. I'd say that's probably good enough. If you want better than that, 0.5% accuracy on the '431 only costs another $0.20 or so, getting you to 1.5% overall. Changing to 0.1% resistors adds around $2.00 and gets you to 0.67% overall. If it were me, building one or two units, I'd be pretty tempted to throw a couple bucks at it and improve the accuracy. Or I might put a in multi-turn trimpot instead of more accurate parts, and make it as accurate as my ability to adjust the pot.

 

[xul]

Are you assuming the resistor values are normally distributed about the rated value?

 

[DIWdiver]

Are you assuming the resistor values are normally distributed about the rated value?

Yes. They're usually much better than that.

 

[The_bad_Frag]

Hmmm ok. I will order that voltage monitor then and try my luck.

 

[moderator007]

Post a close up pic for all to see. Thanks for being the guinea pig, The bad frag. :thumbsup:

 

[The_bad_Frag]

for being the guinea pig

:sweat:

 

[The_bad_Frag]

:mecry:

I saw that the 1,25V version of the TL431 is called TLV431. Somehow only digikey.com has them but they have 18€ shipping!

The 2,5V version is available in a lot of german shops... ...can it work with the 2,5V version too?

 

[alpg88]

oops, didn't notice, it was already mentioned

 

[DIWdiver]

:mecry:

I saw that the 1,25V version of the TL431 is called TLV431. Somehow only digikey.com has them but they have 18€ shipping!

The 2,5V version is available in a lot of german shops... ...can it work with the 2,5V version too?

It depends on the LED you use. The forward voltage of the LED needs to be higher than 2.5V, but less than the minimum voltage where you want it to light up minus some voltage to drop across the series resistor. You want it to come on at 3.6V and stay on until what, 3.4V? Say you want 0.4V across the resistor, so your LED needs to be between 2.5 and 3.0V.

You might achieve this by using a 2.2V green or yellow diode, and adding a silicon diode in series to boost it to like 2.8V. If you choose a red, you probably need two diodes. Also, with such narrow margins, the LED may not go completely out when you want it to. The design is much more solid with the 1.25V shunt.

As far as the TLV431, Farnell stocks them. I couldn't make out enough German to see if they are stocked locally, but they are probably in England. An AP432 is almost the same thing. For low voltages, the LM185/285/385 would work, and the adjustable versions of the LM4041 would also work, as would the NCP100, though each of those is a bit different. I also found ZR431, ZHT431, SPX432, TS431, and TLVH431 varieties listed, and that was just at Digikey.

Surely you could find one of those! If even that doesn't work, ask them if they have an adjustable shunt, or an adjustable reference diode, with a minimum output around 1.25V.

Good luck!

 

[xul]

It depends on the LED you use.

Put a bunch of LEDs in series and pass some small current through them. The ones with the smallest voltage drop are good candidates.
Please post your results because almost no manufs publish min. LED voltages. The more LEDs you use the more statistically robust the answer will be.

 

[The_bad_Frag]

I was busy the last days so I didnt bought anything yet. Farnell has them in stock so I will buy them there. :wave:

 

[The_bad_Frag]

My voltage monitor project did an interesting change.


Somehow I didnt managed to order the TLV431 yet. Today I got my Hobbyking package from the customs office. :duh2: I ordered a few things there so I also bought the Hobby King Battery Monitor 1S . You cant do anything wrong because it only costs 2.94$. Damn its exactly what I need! :twothumbs It can do a lot more things than written on the homepage. Here is a pic of the manual.


The blue led starts to flash at 3,65V not 3,7V as written in the manual. Its just 0,05V of my target voltage of 3,6V so YEAH IT WORKS.
No need to build any selfmade circuits which would cost me a lot more. But I need to unsolder that super annoying buzzer. Its really loud (~80db@1m) and it beeps everytime it gets connected to the battery. (I think its a test function)

Thanks to all of you who spend time helping me. :wave:

...and as a little gift a special recommendation from the Frag This Turnigy Laser Guided Infrared Thermometer is awesome. Look at the price! :huh: In Germany you pay ~120$ for a thermometer with the same features. Its really good manufacturing quality (Im german) and the accuracy is as advertised on the homepage. Have fun with it!


...I used 14 smileys in this post...:shakehead

 

[moderator007]

Thanks for posting this up the bad frag.
Well according to the instuctions blue led shines at 3.7v and up.
Blue led flashes at 3.7v to 3.4v.
Red led shines at 3.4v to 3.2v.
Red led flashes at below 3.2v.

Hobbykings descriptions doesn't say anything about leds. Thats really lucky! You didn't even have to mod it for a led.
It comes with a red and blue led already.:wave: