Uni-T UT61E backlight mod with touch sensor [tutorial]

[Maxlor]

Do you have a pic of it showing the backlight in action?
I'm wondering why you were aiming for 20mA for both LEDs. Or did you use white ones? 8mA is enough for the red LEDs. You don't need to use the meter as a flashlight

Well, I do now, see below.

As for the 20mA, I tested it before by just holding the LEDs in place manually, driving them from my lab power supply, and 20mA looked good. The 8mA is sightly dimmer than I would have preferred. It's usable though, so I let it be.

[Nisei]

Nice!
Looks great.

[extreme]

Great mod 
I buy this unit and it will come in a few days.
Please help me for some question.
Can i use green led ? If yes, what ohm and what type of resistance ? (metal,carbon....)
Thank you.
Happy new year !!!!!!!

[hugo]

If anyone cares about the code, let me know and I'll post it.

I'd like to see it please ...

Thanks

[extreme]

Great mod 
I buy this unit and it will come in a few days.
Please help me for some question.
Can i use green led ? If yes, what ohm and what type of resistance? (metal,carbon....)
Thank you.
Happy new year !!!!!!!

Please, someone can help me ?
Thank you

[mariush]

Yes, you can use any color led, though it would be preferred to use one that requires less forward voltage (like, almost any color besides blue or white which typically need at least 2.4-3v to light up).

It can be any resistor type and the value of the resistor you pick based on how many leds you use (one or two), the voltage you use and the forward voltages.

For example, assuming 5v power , two green leds with forward voltage of 2v and you want 10mA (0.01A) then the resistor value is  (5v - 2x2v) / 0.01a = 1v/0.01a = 100 ohm  ... You'll have two green leds and a resistor, all in series, one chain.

The power wasted on the resistor itself would be P = IxIxR = 0.01 x 0.01 x 100 ohm or 0.01w so any resistor rated for 0.125w or higher would be enough.

If you use 3.3v, then you won't be able to connect the leds in series, you'll have to use one resistor for each led (connected in series to it like in the pictures above) and connect the led+resistor sets in parallel to your power supply. 

In this case you'll have  R = (3.3v - 2v ) / 0.01 = 1.1v / 0.01 = 110 ohm , so you'll probably use 110 or 120 ohm resistors, both standard values  (120ohm would result in slightly less than 10mA current)

[extreme]

Thank you for reply, you was very clear.
I bought a mix of smd led with below data sheet:

3528
Emit color: White
CCT: 5350-5900K   IV:  1000-1300MCD   VF:3.0-3.3V

Emit color: Yellow
CCT:587.5-590NM  IV:100-180mcd  VF:2.0-2.2V

Emit color: Green
CCT:511-514NM   IV:   700-1000MCD    VF:3.0V-3.3V

Emit color: red
CCT: 620-625          IV: 100-150        VF:2.0-2.2V

Emit color:Blue
CCT: 460-465NM   IV: 180-300MCD     VF:3.3-3.6V

Emit color: Orange
CCT: 600-625NM  IV:1100-1300mcd  VF:2.0-2.2V

So in series i think go only red , orange and yellow colour, while in parallel the others.
Is it right ?
Which is the right amp with 2 leds for this project (10mA-15mA-20mA) ?
Thank you

[mariush]

There is no right current, there's only a maximum current for them. In the datasheet, it should say how much current the led can handle and for how much time -
There should be one line saying something like Peak Current 200mA for 0.1s  or duty cycle 1% meaning something like 1ms on, 999ms turn off to allow to cool off. This tells you that you could design something like a camera flash if you need it, but it's obvious that's not the case here.
There should also be one line saying the maximum current in a normal way, if the led is turned on 24/7, it's a smaller value. For SMD leds, it's normal to see values like 20mA , 50 mA etc

This doesn't mean that you should design you circuit for this maximum value because the leds may be too bright with so much current or, the amount of brightness change between 10mA and 20mA may be so small you wouldn't notice, in which case it would be smarter to go with the lower current value.
You have to experiment and see how bright you want the back light to be. Resistors are really cheap, so I would recommend starting with 5mA, then going with 10mA and if you're still not satisfied, maybe go for 15-20mA. It's easy to replace resistors.

I'd suggest trying those orange leds first. I have a hunch they may be quite bright even at very low current values.

[Maxlor]

If anyone cares about the code, let me know and I'll post it.

I'd like to see it please ...

Thanks

Here you go: http://maxlor.com/files/UT61E_Backlight.zip

Some pointers as to what's going on:

• The files in mcc_generated were generated with MPLAB Code Configurator (it's hidden under Tools/Embedded in MPLAB X). The only thing I added manually were 3 lines in TMR0_ISR().
• Measurements are done using the PIC12F1822's builtin Capacitive Sensing (CPS) module. It charges/discharges the measurement cap (my copper strip) using a small constant current source/sink.
• TMR1 is driven by the CPS module, so every charge/discharge cycle increments TMR1.
• TMR0 is configured to trigger approximately 8 times per second. It takes the current TMR1 value, stores it in capCount, then resets TMR1. So capCount contains my cap cycle frequency (divided by 8.) This happens in the TMR0 interrupt, outside of the main loop
• The main loop waits until capCount has been changed from its default of 0. When it has, it compares the value to lastCapCount. If capCount is smaller, that means the capacitance has increased, maybe because the user put his finger on the sensor. At the end, capCount is set to 0, so we notice when it's next updated (by TMR0_ISR).
• The trigger condition takes two successive cycles into account, because the detected capacitance change might be distributed over two measurement cycles. Using only two means that slow changes are ignored.
• Finally, "-100" is the sensitivity; increase the value (i.e. move it closer to 0) to make the detection more sensiive.
• The chip runs on the 31kHz LFINTOSC. It's fast enough for this application, and it makes power consumption negligible (it should be on the order of 75uA with LEDs off.) If you're keen, use the LF version of the chip and disable the watchdog to get to about 9uA.

[joajas]

Hi Nisei,

Thanks for the great tutorial.

I am facing an issue with the triggering, I don't know if you have the same issue.

1. The backlight gets turned on and off when taking measurements with the UT61E. And sometimes after this the backlight will not turn on when I touch the foil area.

2. If I turn the DMM before plugging in the probes, the action of plugging in the probes might turn on the backlight. Especially so when plugging into the COM port.

When it works, it works beautifully, when it fails, its kinda frustrating.

Any idea whats wrong?

[Maxlor]

That is likely caused by the wire leading to your cap being too close to some other components. Keep it short and away from the piezo speaker and digital circuitry. Some Experimentation might be required.

On my first attempt, I had looped the wire around the piezo. Every beep would reliably toggle the backlight :-)

[joajas]

That is likely caused by the wire leading to your cap being too close to some other components. Keep it short and away from the piezo speaker and digital circuitry. Some Experimentation might be required.

On my first attempt, I had looped the wire around the piezo. Every beep would reliably toggle the backlight :-)

Thanks Maxlor!

After some troubleshooting, I found that the sense wire was not properly connected to the aluminum foil that was stuck to the top of the meter. Fixed that and now its working reliably, not more false triggers and lock outs.

[gyrex]

I'm a real dummy when it comes to electronics but I've since been bitten by the bug and am excited about exploring new electronics projects

Anyway, I'm going to attempt this mod on my DMM but have a quick question - how are you guys connecting the aluminium to the wire? I've heard that soldering to aluminium is a real pain in the backside? Also, is it possible to use some stock standard aluminium foil taped down rather than aluminium tape?

I've ordered all the parts and while I'm waiting I thought I'd ask. Thank you to the OP for posting this mod!!!

[MrSlack]

Alu is easy to solder to. You just have to get the oxide layer off first and it's no more difficult than anything else.

[gyrex]

Alu is easy to solder to. You just have to get the oxide layer off first and it's no more difficult than anything else.

Thanks! How does one get the oxide layer off? With flux? Sorry for the stupid questions but as I said, I'm learning

[MrSlack]

There are no stupid questions, just stupid answers

I use a bit of wet and dry sandpaper but you can drag a knife edge over the surface a few times and rotate the wire at the same time to scrape it off.

[TacticalSnacks]

Thanks for the awesome tutorial! I was in the need of a new DMM, saw this mod and used it as an excuse to learn about transistors and touch sensors. I may have gone a little overboard. Pictures and such here goo.gl/pAmFpu. Boards are shared on OSHPark.

I still have the auto power off to finish, but I really want to start using my meter

[MrSlack]

There are no stupid questions, just stupid answers

I use a bit of wet and dry sandpaper but you can drag a knife edge over the surface a few times and rotate the wire at the same time to scrape it off.

Late reply but I use a kitchen scourer pad to do it.

[Fcolon76]

Hi, could you post pictures of the inside of your finished MOD? I'm just asking because I have no idea how to shield the circuit board. I would love to see your pictures so I can try to duplicate yours. Please. Also doing this MOD will it mess up your multimeter readings??? Thank you for your time hope to see those pics! 

[Nisei]

Hi, could you post pictures of the inside of your finished MOD? I'm just asking because I have no idea how to shield the circuit board. I would love to see your pictures so I can try to duplicate yours. Please. Also doing this MOD will it mess up your multimeter readings??? Thank you for your time hope to see those pics! 

I will soon because I'm going to make some changes. I'll take pics when I've opened it.

[Nisei]

Thanks for the awesome tutorial! I was in the need of a new DMM, saw this mod and used it as an excuse to learn about transistors and touch sensors. I may have gone a little overboard. Pictures and such here goo.gl/pAmFpu. Boards are shared on OSHPark.
I still have the auto power off to finish, but I really want to start using my meter

Wow, that's awesome! Wish I had the skill to do that.
Maybe you'd like to know I've altered the tutorial now:
Instead of a 5V regulator I'm now using a 3.3V regulator and have wired the LEDs in parallel.
The reason: the regulator needs an input voltage that's around 2V higher as its output voltage.
So the 5V regulator will need 7V to work.
However, the UT61E can still work below 7V. This means that once the battery gets below 7V, the meter still works but the LEDs start flickering since the regulator doesn't get a high enough voltage.
Using a 3.3V regulator will only require 5.3V input voltage to work reliably so you can use your battery much longer while still being able to use the backlight.
When using a 3.3V voltage regulator you should wire the LEDs in parallel because in series you end up with only 1.65V per LED.

[Nisei]

Anyway, I'm going to attempt this mod on my DMM but have a quick question - how are you guys connecting the aluminium to the wire? I've heard that soldering to aluminium is a real pain in the backside? Also, is it possible to use some stock standard aluminium foil taped down rather than aluminium tape?
I've ordered all the parts and while I'm waiting I thought I'd ask. Thank you to the OP for posting this mod!!!

You're welcome
No need to solder the wire to the tape. Just take 2 pieces of aluminum tape, both sticky side down, and sandwich the bare end of the wire between them.

[Martini]

I don't own a UT61E but read the topic with interest and thought I could share an idea with you:
Why not build a capacitive swipe-switch to choose how bright you want that backlight to be (through PWM)?

[TacticalSnacks]

Thanks for the awesome tutorial! I was in the need of a new DMM, saw this mod and used it as an excuse to learn about transistors and touch sensors. I may have gone a little overboard. Pictures and such here goo.gl/pAmFpu. Boards are shared on OSHPark.
I still have the auto power off to finish, but I really want to start using my meter

Wow, that's awesome! Wish I had the skill to do that.
Maybe you'd like to know I've altered the tutorial now:
Instead of a 5V regulator I'm now using a 3.3V regulator and have wired the LEDs in parallel.
The reason: the regulator needs an input voltage that's around 2V higher as its output voltage.
So the 5V regulator will need 7V to work.
However, the UT61E can still work below 7V. This means that once the battery gets below 7V, the meter still works but the LEDs start flickering since the regulator doesn't get a high enough voltage.
Using a 3.3V regulator will only require 5.3V input voltage to work reliably so you can use your battery much longer while still being able to use the backlight.
When using a 3.3V voltage regulator you should wire the LEDs in parallel because in series you end up with only 1.65V per LED.

I can't take credit for truly discovering the regulator I used (I'll thank Felix Rusu over at lowpowerlabs.com) but the Microchip MCP1703 low drop out regulator will still put out 5V when it's getting 5.7V from the battery. It also has an ultra low quiescent current so when you're not using the backlight, the regulator is only pulling a few uA's.

[Mike60]

I order MCP1703 - 4V, TTP223 ( two different boards ). I receive MCP and one TTP223 ( red board ).
From TTP223 it is enough output amperage to turn on two LED's in parallel ? I suppose NO. In specs are 8mA !
I will try on bench before installing on to UT61E.