4 digit, 12v or less Rpm circuit

I

ID Miller

New Member
I posted some time ago about a tachometer/spedometer got a few good circuit's to attempt. But it's not really what i am looking for. it was a design for a speedometer that counted rpms of a tire. for a car. for a little bit about what i am looking for.


The application.
is in some R/C toys i have. mainly motors and gear boxes. I'm doing my best at learing gear ratios but i get mixed up with the drive driven fractions. I would like for something fast and simple to glance at.

i would prefer to use a 9 volt battery for a power source.

It would be a 4 digit read out that might peak around 9k rpms.

I recentally purchased a new drill press and it has an rpm indicator like i am describing but i do not want to tear apart my press .... maybe in a few years. any insite to this would be wonderful. all i need it to do is drive a 4 digit leds and tell me how many times a min it's counting.

thanks
 
You can use an optical or hall effect sensor, that is read by a PIC or some other microcontroller to drive the display. It will require some knowledge of microcontrollers.
 
If you are not knowledgable with microprocessors, you could do it with a 4 digit counter with displays. I don't know where you live, but there are kits available in Australia to do this.

Then all you need is a one minute timer (a 32kHz crystal and counter), some control logic and an interface for the optical or Hall effect detector.
 
Yeah i'm a total noob when it comes to microcontrollers. I figure their must be a way to do this with out having a controler? and i'm thinking about using an ir detector to detect my revs. i know how i can build a counter no problem there. interfacing it to a 7 seg led though..... is another. 4017 decade counters and 555 timers are what i have worked with the most anything else is sketchy to me. thanks for the input
 
The 4017 is not suitable for driving 7 seg.

An Ic such as the 4033 would be better. See https://www.electro-tech-online.com/threads/4033-7-seg-counter.20028/?highlight=counter

Search the forum for "counter" and you will find plenty.
 
Drill press

Ok so I decided to pull the rpm read out out of my drill press. first off good news it's got a 9v powersupply but the transformer thats next to this board says it's 9vac which is depressing. but it plugs right into this board. this board has a power input and a laser output and emitter/collector connector. so if i'm correct it needs to have a ac to dc converter somewhere on the chip or could the transformer be read wrong. here is a list of every thing thats on the board. every thing is labled with a silk screen except for the ic which is sanded on the top i'm assuming to remove any of the identifying marks.

7 - 150k resistors
1 - 10k resistor
1 - 470k resistor
4 - 15k resistors
5 - 1n4007 diodes
1 - 7530 transistor?
4 - 9012 transistor?
3 - 104 ceramic caps
2 - 30 ceramic caps
1 - 223 ceramic cap
1 - 12 mhz crystal (12.000 KDSY 5B)
2 - 220uf caps 16v
1 - 1uf cap 116v
1 - 20 pin chip of unknown markings (pcb has XTAL next to it)
1 - power connector
1 - 1 emitter/detector connector


could someone look at the picture and make a guess as to what it might be?
it came out of a ryobi 12 inch drill press.
 

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The IC looks like a microcontroller of some kind. It may be a RYOBI special.

So you would have to buy a comercially available PIC and do your own programming.

Alternatively, do it with counter ICs.
 
ID, nearly everything electronic runs from DC -- radios, TVs, computers -- and AC is rectified to one or more DC supplies for the electronics. So never let what appears to be an AC supply trip you up.

Dean
 
converter?

Ok so i got to thinking ... D1, D2, D3, and D4 not act like a converter? like maybe a super basic one? looking at my diagram for a rectifyier it has 4 diodes in it almost similiar to the way this hooks up am i wrong thinking about this?
please reference picture in before post
D1 -D4 is on the right hand side next to the white power input connector
 
D1 - D4 look like a rectifier bridge. So these, in conjunction with an electroylitic smoothing capacitor, rectify the AC to DC.
 
Last edited:

Actually, you can learn a lot from that picture.

First off, the 4 bjts on the left drive the 4 digit display, therefore it must be multiplexed, as there is only 7 segment resistors. The xtal also is a key indication that this is a microcontroller. It may even be a PIC! The diodes on the right indicate that it accepts an AC source, with minimal filtering. There you go..
 
Why don't you just go down to your local automotive/electronics store and buy a non-contact tach? Why re-invent the wheel when you can just go out and buy one, check e-bay for non contact tach, you'll find laser tachs for less than 30 bucks. The first one I found cost 23 dollars plus 10 bucks shipping went from 0-100k rpm and was accurate to .05% Seems like an easy purchase choice to me.
 
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