Frequency to voltage board

**sxtnitehawk** · 03-09-2009, 11:44 PM

awesome... though could you possibly still hit me up about it cause some of the things i want to control are ground switches, not power switches (like my hvac fan, power is constantly applied to the blower and corresponding resistor blocks, it's the switch that completes the circuit to ground...)

**greenman100** · 03-10-2009, 08:44 AM

you could use a npn transistor i guess

what's your aversion to relays?

**FredWorrell** · 03-10-2009, 06:16 PM

Most excellent. Even with +/- 40 rpm, if I can change a resister, I will do that. I was going to build my own tach board, but now I will wait for this. BTW this will be part of my glass panel for a Dragonfly, experimental aircraft.

Fred

**sxtnitehawk** · 03-10-2009, 06:42 PM

Originally Posted by greenman100

you could use a npn transistor i guess

what's your aversion to relays?

they click...........i HATE clicking sounds while i'm driving.....

How are you designing the driver board? power in and power out for each driver, or will it have a common power? If it's in/out, then it'll work just fine for me for both power and ground switching...

oh, and they're mechanical..... IMHO Solid State > *

**greenman100** · 03-10-2009, 07:37 PM

it's a MOSFET.

if you put these relays in the dash or engine bay, you won't hear them... at least in my experience.

**greenman100** · 03-27-2009, 06:00 PM

Just got these in.

The manufacturer botched up, they're 210Hz/volt instead of 67Hz/volt.

I just did some preliminary testing with a waveform generator. It handles sine, square, triangle waves with no change. I tested 50mV to 5V signals, no problems.

Accuracy is about 2 Hz from 10Hz to 650Hz.

You can solder in resistors of your own choosing to change the range, but be advised, it's SMD. It's not impossible, but you'll want a fine tipped iron for sure.

On the next revision, we'll either socket the resistor, or make it full size.

**FredWorrell** · 03-27-2009, 06:24 PM

Do you have an algorithm to size the resistors based on desired range? I will break out my tiny soldering iron.

**greenman100** · 03-27-2009, 07:18 PM

well, it's a product of range and input voltage. it's pretty simple as far as I can tell.

Vout= Vin * R * C * Fin

so, in our case we have 5 *100000*.01*10^-6*210

(a 100k resistor and a 01uF cap)

which gives us approx 1 volt per 210Hz, which is what I'm seeing.

The problem is, the circuit was initially designed for 15 volts, and we're using it with 5... so everything dropped by a factor of 3.

You can supply it with 15vDC, or change the resistor as you see fit. You need to increase the resistance. On the board, what I'd recommend doing is cutting the trace on the GND side of what's labeled R2 in the attached board layout. Then, solder to what is labeled pin 3 on U1. Install the new resistor between pin3 on U1 and ground.

Ugh.

**FredWorrell** · 03-27-2009, 07:53 PM

Excellent. So I will be replacing R2 with my resister. I want roughly 1 volt per 1000 rpm, or 1 volt per 33hz. Should be about 600K. I want to keep my supply at 5 volts to prevent accidental over voltage to the brain input.

So how do I buy??

You are really good at this. Have you thought about a thermocouple amp (possbily using the AD595 chip)??

**RonS** · 03-27-2009, 08:05 PM

Yes how do we buy. And also if I am calculating correctly. If I put a 1 meg resistor in. I should get 5 volts out at 6000 rpm. 5 *1000000*.01*10^-6*100 I was just looking at the board and if I take the resistor off and drill a hole in each pad. Then I could put in a 1/4 watt resistor standing up. Or just surface mount it standing up I guess.