# 2 digit meter with double lm3914

Discussion in 'The Projects Forum' started by jtelectric, Aug 27, 2013.

1. ### jtelectric Thread Starter New Member

Aug 26, 2013
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0
I am going to be setting up a variable speed pump. The speed of the pump will be controlled with a 0-5v output on a temperature controller. I want to set up a meter using 2 LM3914, 2 74LS147, and 2 4511 ICs. I have looked at a lot of posts, projects, and data sheets. what I'm not sure of is how to set up the 2 LM3914 ICs and interconnect them to the 74LS147 ICs. If someone can please help me.

2. ### jtelectric Thread Starter New Member

Aug 26, 2013
4
0
I would like to display 00 to 99 % speed of the pump

3. ### JohnInTX Moderator

Jun 26, 2012
2,315
985
What you are asking for is to cascade 2 LM3914s to get 10's and 1's but they don't work that way. Cascading 2 LM3914s gets 1-20, not 1-99.

I would use an off the shelf DPM module with a suitable voltage input and scale it appropriately.

Something like these from Mouser. Or Red Lion from Digikey. Alternatively, you roll your own with a uC or dedicated DPM IC (Intersil used to make these, maybe still do).

Good luck!

Last edited: Aug 27, 2013
4. ### jtelectric Thread Starter New Member

Aug 26, 2013
4
0
I work with them all the time. I wanted to build something my self. That's why I was thinking a decimal to BCD converter and BCD to 7-segment converter would give me my 00-99%.

5. ### JohnInTX Moderator

Jun 26, 2012
2,315
985
If I get what you are trying to do its like this..

Run the first LM3914 in the dot mode.
Feed that 1 of 10 output into the 74LS147 to convert 1 of 10 to a BCD digit.
Feed the BCD digit into the 7 segment display.
Duplicate for the 10s digit.

That works for one decade but consider that your 2 cascaded LM3914 goes from 1-10 on the first one and 11-20 on the second one, each 'count' being an equal voltage increment. You are wanting the first LM3914 to recycle to 0 when LED11 turns on, then leave LED11 on while LEDs 1-10 count up again, turn LED12 on and recycle LEDs 1-10 again and so on for each decade. It doesn't work that way. Two cascaded LM3914s have only 20 unique indications not 99. Once your input is >10, the first LM3914 is pegged and that's it.

To make it work like a 2 digit counter, your analog inputs would have to be different with the high digit going up by 10 and the low one going up as the remainder of the input divided by 10. In analog. Kapish?

6. ### jtelectric Thread Starter New Member

Aug 26, 2013
4
0
I see what you are saying now. I didn't think of it that way. Then I might just do 20 LEDs then