saiello Posted January 8, 2007 Report Share Posted January 8, 2007 Hi All, I've almost finished my project for a tachometer circuit. Testing went well until I appear to have hit upon a snag for which I need some help. It concerns the LM2917N frequency to voltage converter IC, which I can happily 'program' to give the correct output with a given frequency input ( 0-2500Hz ) using a frequency generator. The problem arose when I tried to vary the 'duty cycle' of the input frequency ( which is a square wave ), i.e. I adjusted the 'HI' time to make it a smaller percentage of the time 'LO'. Whatever frequency I input, if I adjusted the HI so that it was less than 0.1ms ( 0.0001s ), the voltage output from the LM2917N started to drop. I've tested the LM2917N with higher input frequencies with a Hi/Lo duty cycle of around 50% giving half-cycles much less than 0.1ms, and it still outputs a voltage ( although off the scale ). It is important that the LM2917N can accept low duty cycle input frequencies as it will be generated from a laser diode/phototransistor pair reflected off a rotating body where duty cycles can be as low as 1%. I've also tried testing using the laser diode/ phototransistor as the input in a 'real' test situation, i.e. reflecting off a rotating nut. With a given output voltage if I gradually block off the light recieved by the phototransistor, I start to get the same erroneous result, i.e a drop in output voltage, indicating that the reducing duty cycle is to blame. I've checked the datasheet for any issues concerning this but I can't seem to find anything relating to this. Has anyone had any experience of this? Any help much appreciated! :)Thanks,Salvatore. Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 8, 2007 Report Share Posted January 8, 2007 Hi Salvatore,The datasheet talks about selecting R1, C1 and C2 values for a required response time. Quote Link to comment Share on other sites More sharing options...
saiello Posted January 8, 2007 Author Report Share Posted January 8, 2007 Hi audioguru! I remember you helping me out on my comparator problem! Quote Link to comment Share on other sites More sharing options...
saiello Posted January 9, 2007 Author Report Share Posted January 9, 2007 Hi, I think I've found the problem! :D There's a formula in the datasheet for determining the maximum frequency that can be input: Fmax=I2/C1*Vcc=0.00018/(0.0000000047*7.57)=5060Hz. At this frequency, the half cycle pulse width would be 0.098ms which from my testing turns out to be just about the value that I determined was starting to give me problems. So the only solution I can see is to increase the pulse width at source, i.e. somehow delay the pulse generated by the phototransistor so that it remains a constant at around 0.1ms regardless of the input frequency or duty cycle. Anyone know how I might go about doing this? Maybe somehow using a capacitor on the input to the comparator to store the charge for a short time? I've attached a diagram of the input stage to give a clearer idea.Thanks,Salvatore. Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 9, 2007 Report Share Posted January 9, 2007 hi Salvatore,I thought the values of the resistor and capacitors of the IC will solve your problem.A monostable "pulse-stretcher" can be made from a 555 or a couple of Cmos inverters. Quote Link to comment Share on other sites More sharing options...
saiello Posted January 9, 2007 Author Report Share Posted January 9, 2007 Hi, If you can determine what values for R1/C1/C2 I need to solve the problem please let me know! :D Will the cmos inverters just invert the signal? If this is the case then it won't solve the problem as a Quote Link to comment Share on other sites More sharing options...
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