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

[tim_the_toolman]

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]

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]

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]

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]

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]

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