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New guy with a simple question about the LM3914 IC...help?

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tim_the_toolman

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I am trying to figure out how to create a circuit that will monitor voltage in DC (automoile) from 0 volts to an adjustable point, say 5 volts, and display the voltage in a LED display. But also trigger a relay for a add-on accessory at a given voltage. I have come across the tech sheet for the LM3914 IC and it looks like I may be able to use the chip to aid in my creation but I need a little guidance. Can I make the bar graph adjustable with a pot or somthing? Where in the circuit would it go? Does this chip need separate power ( which would be 11-15 vdc by the way )? Would the LM3915 be more suited? How can I set the trigger point for the relay, or is that a separate circuit all together? Please help :(
 

Jaw174

New Member
what do you mean by adjust the bar graph? do you want to adjust the brightness, or how many segments it controlls. please specify.

as for triggering after a set voltage, use a (whatever voltage level) zener diode with output to an NPN transistor, as a simple way to do it.
 

tim_the_toolman

New Member
Well I will be using this meter to monitor the voltage from a throttle posistion sensor on different vehicles. On some vehicle the TPS voltage runs from 0 - 5 volts and others run to more or less than 5 volts so I wanted to be able to adjust the peak voltage point so that the led's reflect the peak voltage at wide open throttle. IE if the car had a tps that ran from 0-8 volts the 10th led would light at the 8 volts. But if I moved it to a car that had a range of 0-3 volts the unit would have to be adjusted to reflect three volts at the 10th led.

"as for triggering after a set voltage, use a (whatever voltage level) zener diode with output to an NPN transistor, as a simple way to do it"

Could you explain this concept a bit more? Since the peak voltage of the main unit needs to be adjustable so would this trigger point. So at wide open throttle it would close a relay for example regardless of what car I am using it on.
 

Klaus

New Member
Tim, the 3914 is a linear device, the 3915 a logarithmic. The latter is more useful for audio level monitoring.
The LM3914 chip as is goes from none to all 10 LED's lit when an input at pin 5 (18 pin DIL packake) goes from 0 to 1.2 V. You need a voltage divider to drive it from higher voltages.
In your case, you would have to adjust this divider for different input ranges, use a trim pot (or several switch selected ones) as a voltage divider.
You can use either bar or dot mode, bar mode dissipates more power of course.
I used three chips cascaded for a 30 LED tacho display, ran the chips & led's from a 5 V regulator to avoid the hassle with fluctuating battery voltages.
Klaus
 

tim_the_toolman

New Member
thanks for your reply...so if i want to make this adjustable i would have to put a trim pot on pin five? as far as the trigger for a relay goes the lm3914 tech sheet says i can run a transistor type switch along with the led to switch a separate crcuit....any input on how to calculate what transistor. also are there any tables out there that show what resistors to use to change the range on the lm3914?
 

Klaus

New Member
Yes, run your input to one side of the trimpot, the wiper to pin 5 and the other end to ground.
Tis IC sinks current to turn on LED's. There's no reason you could not substitute a LED with a suitable relay if you put a reversed diode across the coil. If you want to drive a relay in parallel witout dimming the LED or overloading the 3914 you need a little circuit there to get some drive voltage for a transistor.
I have here some application notes, maybe they are on the net, do a Google search for 'LM 3914 application notes', using the exact words advanced search feature, perhaps it turns up somewhere.
Its not hard to do if you know a little about transistors.

Klaus
 
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