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# extremely basic analogue calculator

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#### grrr_arrghh

##### New Member
Hi

I had an idea for a very simple analogue calculator.

several variable resistors in series, each is turned to an ammount, the total resistance would be the sum of the three ammounts. Its pretty basic, but its something to fill my otherwise empty life

but, my only stumbling block is how to measure the overall resistance, without using a multimeter (which would defeat the point a bit)

i'm thinking either seven sigment displays, or an analogue ammeter, but i'm not sure how to 'invert' the current (under normal circumstances, the higher the resistance, the lower the current)

maybe it could be done with a voltmeter?

all suggestions welcome

Tim

It's called an 'analogue computer', it's what opamps were invented for - way back before IC's. A meter was commonly used for a readout.

oh...

hmm, op-amps make some sense, although it'll take me a while to think how to do it.

if i remember, epe mag did a feature on an analogue computer - this was quite complex, is this what i am going to end up trying to make (if so i'll give up now!!), or is there more simple versions that i could make?

Thanks

Tim

It depends how far you want to go, the EPE was quite limited as I recall.

The technique with an analogue computer is to simulate the function you are trying to calculate, it's where many of the opamp circuits (mixers, integrators etc.) originally came from.

Because of the analogue way they work, you generally need to wire a number of different sections together - this is why analogue computers were usually provided with lots of plugs and sockets. With a digital computer you write a program which goes into memory, with an analogue one you actually construct a circuit which does what you want.

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oh, ok thanks

i think i could probably work out some other functions out using that kind of circuit.

limited or not, that was all i was really trying to create, i just wondered if there was a better way of displaying the output (than using a voltmeter)

thanks again

Tim

i just wondered if there was a better way of displaying the output (than using a voltmeter)

You can use a LM3914 IC (or a cascade of these) that show the voltage level on a series of LEDs.

interesting - thats what i'm talking about

but - how?

and ermm it seems that the LM3914 has been discontinued - is there a good alternative?

thanks

Tim

Hi, again

I constructed the operational adder that you posted (a snake that can do maths, how comedy!), using input volatages of between 5v and 10v, and the voltage at the output was only around 80mV ?????

I has assumed that if all three input voltages were 5v, then the output would be 15v (5+5+5) ?

maybe i missinterpreted what you meant? :?

It would actually be -15V, because the circuit inverts, to correct it you would add another opamp as an inverter on the output. Obviously the output voltage is dependent on the supplies to the opamp, and it's capabilities. Just as with a digital adder you have to avoid overflow, with an analogue one you have to keep within the supply rails.

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