Hi,

I am doing a project for university which required me to build a tachometer for a dc motor I built. I have so far set up a circuit with an LM2917 and LM3914 to LEDs. If I input a signal from a signal generator into the first pin of the LM2917, more LEDS light up for more frequency as it is suppose to be. I am now up to building the sensor for my motor. I have set up a reed switch however it pulses from 0volt to 3volt. I now think from the reading Ive been doing the 2917 needs a negative voltage as well like -1.5 to +1.5. I am using two 1.5V batteries for the reed switch so is there anyway to wire this like that or will I have to go with a different sensor. I am very new to this so sorry if some information is wrong. I tried doing the batteries like the attatched picture but dont know how to wire it to the reed switch.

Thanks

 

Hello,

As far as I know a reedswitch is a mechanical part.
Mechanical parts will fail at high speeds.
Perhaps take a look at a hall sensor.
Here is a page with informative links on hall sensors:
http://www.educypedia.be/electronics/sensorshall.htm

Perhaps you can take a hall senor from a bad working DC fan.

Greetings,
Bertus

 

Thanks for your reply.
Yeah I have a hall effect sensor and I also bought an opto coupler so I have almost every sensor I could think of and I am willing to set up whatever however I believe the same signal will come from all of these which is a 0V to whatever and I think the lm2917 needs - to + V. Is this right, if so can you get it from these sensors.

Thanks

 

Hello,

The voltage needed for the lm2917 depends on the model. (see datasheet).
The 8 pin version has one input tied to ground.
The 14 pin both inputs are free, so you can set a detection level.

Greetings,
Bertus

 

The LM2917-8 requires a +/- signal with respect to ground. The LM2917 does not. Check the first two paragraphs on page 7 of the datasheet: http://www.national.com/ds/LM/LM2907.pdf

Ken

 

yeah I read that and from all the reading on these forums I know the 14 pin can accept a pulsed DC however how do you wire it like that.

 

Take another look at the datasheet. Pin 14 is "NC"..."not connected" to anything. The input opamp is acting as a comparator. Set pin 11 at some positive voltage that is about half way between zero volts and the peak positive pulse voltage of your input transducer signal on pin 1.

Ken

 

A common form of a Hall-effect senson appears like a NPN transistor, with magnet close output is low. Connect as an emitter-follower, the output is ground to almost B+, so should be able to use either 8 or 14 pin .

 

so should be able to use either 8 or 14 pin .

?????? On an LM2907 pin 1 is the input, pin 8 is the output transistor's collector and pin 14 is N/C.

Ken

 

Was referring to no. of pins on package, as LM2907M-8= 8 pins, LM2907M= 14 pins. As long as input signal goes to ground either version will work.

 

Right. But I think, from the datasheet, that on the LM2907-8/LM2917-8 the input has to swing negative with respect to ground, because the non-inverting input on the input amp in fixed at ground potential.. Check post #4 and #5.

ken

 

" so that an input signal must swing above & below ground." Crow is'nt chicken but not bad.

 

thanks for all your replys guys. I eventually figured out how to do it. I am triggering the 2917 off a reed switch and wiring two batteries and center tapping it gave me that negative to positive swing the 2917 needs and it works a treat.

I do have another problem though. I amhooking this up to a lm3914 which works and I hooked it up according to their schematic which should give 0-5V signal in to allow all the lights to light up. I am sure I have wired it correctly and using the same resistor values(checked twice) I dont get that 05V range i get about 0-1.3V. I have tried different resistors but the 0-1.3 stays the same. This is not a problem because this is the range I have calibrated the LM2917 to output according to the max frequency of my motor. However I am trying to write a report and I am just wondering is the minimum range that chip could do is 0-1.25 if the resistors R1 and R2 were the same. This is the data sheet http://www.national.com/ds/LM/LM3914.pdf

Cheers

 

The LM3914N was posted a few weeks ago as a 5V bar-graph meter, not tested.