Using 3 LM3914 chips as position sensors for car pedals

Thread Starter

Blinkity-Blonk

Joined Jan 1, 2022
5
Hello, I have decided to embark on an electronics project for my own enjoyment and gain of understanding of electronics. I'd like to have 3 x LM3914 dot/bar graph chips together to show the position of Clutch, Brake & Accelerator pedals in a car using Yellow, Red & Green LEDs respectively linked to slide pots attatched to the pedals.

I think I have got the basic circuit figured out but there are a few things I am unsure of, so I have joined this forum to hopefully get some help :)

I'd like to be able to switch all 3 between dot/bar using 1 switch and have the ability to dim all 3 using a pot. I am not sure the way I have it hooked up is correct. The switch is easy, but I am not sure how to select values for a pot (slide or rotary). Aside from the resistors on the LEDs, which I've got as 500 Ohm 0.5W (based on an online resistor calculator). In reality I'd use two 1000 Ohm 0.25W in parallel, as I have loads of them to hand. I am not sure of the other resistor values I'd need. I can find quite a lot of info on here and elsewhere for the LM3914 but not a vast amount about running it at 12V.

I have downloaded LTSpice and built up the circuit to where I think it should be. Most of the components have been left 'as is' apart from the LED resistors. I can make the circuit 'run' but as for how to interpret the results I've hit a dead end.

I'd be happy to give it a go and actually build up the circuit on a board but I kinda want to limit the number of things I blow up!

Thanks in advance :) (and go easy on me, it's my first day!)
 

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dl324

Joined Mar 30, 2015
15,459
Welcome to AAC!
go easy on me, it's my first day!
Have you read the datasheet? LED current is set by the IC so you don't need the current limiting resistors.

You show several independent 12V supplies. To enable bar mode, that pin must be within 20mV of the LM3914's supply.
 

Thread Starter

Blinkity-Blonk

Joined Jan 1, 2022
5
Welcome to AAC!
Have you read the datasheet? LED current is set by the IC so you don't need the current limiting resistors.

You show several independent 12V supplies. To enable bar mode, that pin must be within 20mV of the LM3914's supply.
Yes, I've got a copy of the datasheet. Got a bit excited when I realised you could set an alarm on it as well - I'd like it to flash the red when you brake hard!

I was aware the IC set the LED current, but what I've also read is that they get pretty hot so the thought was that by having an external resistor for each LED it'd take some of the load and heat away from the IC.

I know there are multiple 12V supplies, but in reality there will only be one. Just thought it looked neater on the diagram not to have wires everywhere...
 

dl324

Joined Mar 30, 2015
15,459
but what I've also read is that they get pretty hot so the thought was that by having an external resistor for each LED it'd take some of the load and heat away from the IC.
Where did you read that? As long as you follow the guidelines in the datasheet, you should be good to go.

Just thought it looked neater on the diagram not to have wires everywhere...
Can't say that I've ever seen an LTspice schematic that looked "neat".

You can use netnames to avoid some wire crossings. Your schematic would be neater if you avoided unnecessary wire jogs and didn't run wires over component designators or values.
 

Thread Starter

Blinkity-Blonk

Joined Jan 1, 2022
5
It was a post on another forum that mentioned the heat build up of the IC. I think I'll have to print out the datasheet for some bedtime reading.

That sequence readout makes perfect sense and was more or less what I was hoping to see on mine I just haven't yet managed to make it work yet. I expect I've missed something somewhere.
 

ElectricSpidey

Joined Dec 2, 2017
2,182
I don't think you can use the default configuration and set the brightness dynamically without altering the reference high setting at the same time.

But you can probably de-couple those and set the reference high pin with its own divider then use something like a follower circuit to feed the brightness pin. Ref Out (which sets the brightness depending on the current flowing in the pin)

Here is a screenshot of a project I did some time ago setting the reference high with a divider and using a 1k resistor to set brightness.

AAC_3914.JPG

Note that you will need some moderately low resistor values to set the divider (unless you use VCC) because that pin in not exactly low impedance.
 

eetech00

Joined Jun 8, 2013
3,418
Hi

Since VREF is constant, I believe you can just jumper them together and one resistor function as the current limit "set" resistor.
I'm not sure tho..but do have to account for the current split thru the resistor to each device.
I only showed the acceleration signal in the sim below.

1641774039225.png
 

ElectricSpidey

Joined Dec 2, 2017
2,182
There is also an alternative dimming method, that only uses one additional part that being a single NPN transistor.

Set up the 3914 to drive its output transistors to ground (full bright) then dim using the high side with the NPN in follower configuration.
 

Thread Starter

Blinkity-Blonk

Joined Jan 1, 2022
5
Cool, thats all really helpful stuff. I'm not 100% on what slide pots I need for the pedals yet anyway so adjustment there might be preferable. I have all the other bits so I think its probably time to start building.
 

ElectricSpidey

Joined Dec 2, 2017
2,182
You will need a heatsink if you used the above alternate method of brightness control.

As far as the chips getting hot, yes under certain circumstances they will get very hot.

Consider this...20mA each LED @ 2 volts each (red) all on with a VLed of 12 volts, that would be 10 volts across the internal drive transistor times .02A now times that by 10.

Now consider 30mA per LED...:eek:
 

Thread Starter

Blinkity-Blonk

Joined Jan 1, 2022
5
You will need a heatsink if you used the above alternate method of brightness control.

As far as the chips getting hot, yes under certain circumstances they will get very hot.

Consider this...20mA each LED @ 2 volts each (red) all on with a VLed of 12 volts, that would be 10 volts across the internal drive transistor times .02A now times that by 10.

Now consider 30mA per LED...:eek:
Considering my options here... I work with a lot of aluminium so a heatsink would be easy to find, although potentially bulky in limited space.
Alternatively I don't have to use 12V either. I could drop it to 5V just by running it through a usb. That'd have the added benefit of a more stable voltage. Vehicle voltage is a nominal 12 at best.
 

eetech00

Joined Jun 8, 2013
3,418
Considering my options here... I work with a lot of aluminium so a heatsink would be easy to find, although potentially bulky in limited space.
Alternatively I don't have to use 12V either. I could drop it to 5V just by running it through a usb. That'd have the added benefit of a more stable voltage. Vehicle voltage is a nominal 12 at best.
If your not aware, a heat sink can be purchased.
 
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