My 2c worth, For the amplifier, use a low power opamp in open loop, or use a comparator. Either way, youll get a signal with fast edges and flat tops, which is what you want. Jim ----- Original Message ----- From: "Vitaliy" To: "Microcontroller discussion list - Public." Sent: Saturday, August 15, 2009 11:17 PM Subject: Re: [PIC] Bicycle Speedometer Project > Just my 2c: > > - The problem with the reed switch you describe is unlikely to happen in > real life, and can be easily remedied in software. Just check for minimum > time b/w pulses, or implement an even more clever algorithm. I have no > idea > why Olin thinks it is not a good choice (it's certainly good enough for > commercial speedos). > > - If you go with the coil option, you need an amplifier that will take the > pulse from the coil and get it up to 5 volts. Put a low pass filter (in > the > simplest form, a cap) on the input of the amplifier. > > Vitaliy > > > solarwind wrote: > On Sat, Aug 15, 2009 at 7:55 PM, Olin Lathrop > wrote: >> A reed switch is not a good choice. This is usually done with a permanent >> magnet between two spokes of the back wheel as far radially outward as >> possible and a pickup coil on the frame. A little analog electronics and >> you get a pulse into the PIC once per wheel rotation. > > Good idea, but the analog electronics part is wayyyyy too complicated > for me. But I'm going to go for it as I can already see an advantage > with the analog solution: if I were to use a reed switch, it may > rapidly switch on and off if the magnet stalls near the switch. > However, with a coil, it is possible to filter that out. > > This approach also raises a few more questions: > > * What kind of coil would need to be used? Thickness? Resistance? > > * Would a coil pulled out of an analog clock work? (Was formerly used > in the clock to move the seconds hand.) > > * How much coil is needed and how will it be configured physically? > Will it be wrapped around a pencil, for example? > > * I'll look at the signal produced by the coil through an > oscilloscope, but what other electronics is needed to isolate the > wanted signal? Or can a PIC alone and it's A to D converter tackle the > task? > > I'll have to do some reading on analog filter design. > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist