# detect speed of pulsed input ?

Discussion in 'The Projects Forum' started by s8utt, Feb 5, 2012.

1. ### s8utt Thread Starter New Member

Feb 5, 2012
1
0
Hello, I'm a newbie here but I was hoping for a few pointers.

I have a circuit that detects a flashing light off a electricity meter.

What I would love to be able to do it build a circuit that drives a 10 segment led bar. The input would only be a pulsed input, depending on the speed of the pulses depends on how many led's light up.

Lets put a few figures on the table.

If I say that the meter flashes 1000 times for every kWh consumed.
There are 3600 seconds in an hour, so if I was using 4kW's the pulse would be once every 0.9 seconds

3600 seconds in an hour/4000 flashes in an hour = 0.9 flashes per second

It is this rate I want to detect, if we look at the following table

watts..........pulse every sec
3800...........0.947368
3400...........1.058824
3000...........1.2
2600...........1.384615
2200...........1.636364
1800...........2
1400...........2.571429
1000...........3.6
600.............6
200............18

The 10 lines indicate the 10 led bars, if the pulse was less than once every 18 seconds, no led would light.
Between 6-18 seconds, only 1 led would light (bottom one)
between 1.05 and 1.2, 9 leds would light ( from the bottom up )

You get the idea, I want to convert a pulsed input into a bar graph, the faster the pulse the more led's light up

Can anyone give me any pointers ??

Many Thanks

2. ### crutschow Expert

Mar 14, 2008
13,509
3,385
An easy way would be to program a micro to do the calculations.

An analog approach would be to average the pulse value (for a fixed pulse width) with a long time-constant RC filter. This average analog voltage could drive an LM3914 dot/bar display driver. You just have to select the proper RC time-constant and the sensitivity setting of the LM3914 to get the scale factor you want, based upon the pulse characteristics (voltage, pulse width, and rep rate). (You may need to stretch the pulse width with a 555 one-shot if the pulse width is too short to give a good average signal level.)