resistance controlled timer

[steveyg]

Hi all,

Ive been asked to make a circuit that operates a relay for a selectable amount of time.

Using dials like that from a decade box i hope to be able to set the time for how long the relay is 'on' before returning to its 'off' state, it should be operated using a push switch regardless if the switch is still pressed down.

The trouble is I cant find any type of circuit where a timer is controlled by resistance, or multiple resistance dials, hopefully i can make it in the range of microseconds to 10 seconds.

any help please?

 

[tronitech]

You need a monostable based on a 555 timer. Have a look at this for starters.

 

[MikeMl]

How accurate do you need to have the delay?
It takes several tens of ms for a relay to pull-in; even longer to release.

You will not be able to get such a wide time range (1000000 to 1) with just one timing capacitor, so you will have to have a range switch to select the Capacitance, and then something like a precision linear 10-turn pot with a turns counting dial to set the resistance.

Frankly, this is a job for a PIC where everything is done digitally...

 

[crutschow]

If you wanted resistor control with a PIC, you could input a variable voltage from a pot and convert it to digital with a PIC that has a built-in A/D converter. Then use the internal digital output of the A/D to determine the output pulse width. You could make the selection of the width non-linear with respect to voltage, to cover an arbitrarily wide pulse-width range.

 

[KeepItSimpleStupid]

Think about using an knob based quadrature encoder. Typical specs ate about 16 pulses per revolution, a integrated push button switch and even a bicolor light. See www.sparkfun.com.

Store the time value in EE Prom, flash or ferromagnetic memory.

 

[steveyg]

Hey all, thank you very much for your replys!

Ive now been told not to use a 555 because theyre hard to calibrate and i should try for a harder project to make as 555 is deemed too basic, PICs and TTL counters are the route i should take!

It has to be pretty accurate, maybe to 10's of microseconds so the switching of the relay will be a factor so maybe i could start counting after the relay makes contact!

Im now thinking of other ways to control the time other than resistance wheels and may likely use an oscillator

Im not very good at programming PICS either so the simpler the better really

any thoughts?

 

[OlPhart]

Howdy... Warning... SCHOOL PROJECT ALERT (the "I should try for a harder project" is a Dead give away).

Just buy one from IDEC, Omron or Allen Bradley ($!). You can't even design & heat a soldering iron under the cost to buy from them (exc AB).

That it's a school project... Do Your Homework... Bad Hunting..Go Fish <<<)))

 

[crutschow]

If you need tens of microseconds of accuracy then you can't really use a mechanical relay (the contact bounce alone is several milliseconds). A solid-state type relay would be better for better accuracy and repeatability, with no contact bounce.

 

[steveyg]

Again thank you for your replys.

ive talked it over with a few people but the majority seem to think that going down the PIC route may be easiest. i will also use solid state relays to take away the switch time although i see they are still not perfect but i suppose ill get that sorted at a later date

An alternative someone else suggested was an oscillator, TTL counters/dividers, latches, BCD switches and signal gates but i dont quite understand the whole picture, i gathered they were counting each pulse and dividing it until the required amount of pulse occurs calculated from 1/t etc...

But as stated i will try for the PIC route so if you could suggest;

1)what PICs look at first and why

2) how to set the time/counter from an operators point of view i.e. from switches on a box that operates the required count/time. im just unsure on whether i need BCD counters or decimal counters.

i.e. https://uk.rs-online.com/web/p/thumbwheel-switches/4250136/ or https://uk.rs-online.com/web/p/thumbwheel-switches/4250120/

any thoughts

 

[KeepItSimpleStupid]

I'm confused too.

Usually you have Decimal (0-9) or HEX inputs (0-F)
BCD outputs in either complement or non-complement form

I have NO IDEA what decimal output is. It could be a 1 of 10 line. e.g.

You have to get the datasheet. Take a look at www.farnell.com or www.newark.com or even www.digikey.com for stuff with datasheets.

There is also different ways to activate a thumbwheel switch:
1) A lever - it just moves through the positions and is very rapid
2) A push button for increment and 1 for decrement - Not as fast as #1
3) A flip type motion. Generally used for infrequent changes

Now, you might be able to convince the powers that be to use an encoder: See https://www.sparkfun.com/products/10596?

So, if you had a display, you could select the digits and increment/decrement them.

This is no more than a suggestion.

 

[steveyg]

I'm confused too.

Usually you have Decimal (0-9) or HEX inputs (0-F)
BCD outputs in either complement or non-complement form

I have NO IDEA what decimal output is. It could be a 1 of 10 line. e.g.

You have to get the datasheet. Take a look at www.farnell.com or www.newark.com or even www.digikey.com for stuff with datasheets.

There is also different ways to activate a thumbwheel switch:
1) A lever - it just moves through the positions and is very rapid
2) A push button for increment and 1 for decrement - Not as fast as #1
3) A flip type motion. Generally used for infrequent changes

Now, you might be able to convince the powers that be to use an encoder: See https://www.sparkfun.com/products/10596?

So, if you had a display, you could select the digits and increment/decrement them.

This is no more than a suggestion.

Thanks for your reply, no loss about the decimal switch. i think i will try the flip motion/thumbweel selector at first.

im researching encoders now, i can see the use of it but i feel it just may add to the complication of this project, i initially just need this to work, again the same with the display add another (at this point) layer of complication to it, at first i will stick to the thumbwheels and selector that wither move the decimal place or the range i.e. x10-3, x10-6, x10-9

thanks for your reply but im still unsure on how i would use the thumbwheels and connect it to a PIC or a 4017 chip

any thoughts?

 

[KeepItSimpleStupid]

You can do the thumbwheels by scanning. Digit outputs and BCD inputs.

You can even do them with single inputs (4 digits x (4)BCD or use a GPIO expander for an I2C bus.