Perfecting the 555 Timer: Adding an LED Display for Precise Liquid Dispensing

[daveroderick]

I want to first thank all of you for your help with my first 555 timer question. With that help and some more reading on the net I have it working perfectly.

My next task on this timer project was to include an LED display. My intention was to have a display that would give me a numerical reference. What I am going to do is drive a 12 volt relay with the 555 that switches on a liquid pump. The idea is to dispense a certain amount of liquid during the timing cycle. The numerical value for different amounts of liquid will be recorded so the timer can be changed for different amounts of liquid easily.

I have included a 50K PCB mount pot just before my 200k panel mount potentiometer and 2200 micro farad capacitor in the timing portion of the circuit. The 50K pot is for calibration. So, I need to be able to include a display that will give me a numerical value for the 200K potentiometer settings. The calibration pot would be important so I can “adjust” the timing slightly as the pump gets weaker or values in the circuit change over time. The calibration pot would allow me to keep my reference values on the display.

I have been reading about digital displays and how they work and I have not been able to wrap my mind around the concept. I originally thought that there would be a display out there that would take a reading before and after the potentiometer and give me a value. I had expected to be able to get readings like what I see on my ohmmeter when I change the setting on the 200K pot on my work bench. If I understand what I have been reading, I have not ran across a topic on displays or a tutorial that covers what I am trying to do. The book that I have been going through on electronics does not cover displays and how they work. Can anyone direct me to a place I can go or a book that I can buy that would give me direction on this subject? It would have to be a book for “dummies” though. LOL

I hope that I have described what I would like to do in enough detail. And I know that all of you spend more than your fair share of time helping out “newbies” like myself but I would appreciate any help or guidance any of you could give.

Thanks
Dave

 

[vk6kro]

It is time for you to get familiar with Picaxe chips.

555s are OK if you just want a rough oscillator of about the right frequency.

A Picaxe can produce exact crystal controlled delays and pulses that never change.
They can also drive a counter to give a LED display if you like or they could give a serial output which you could use to drive a serial LCD.

The programming circuitry is just two resistors and a serial cable so the resistors can be left on the circuit board and the cable plugged in as required.

And they use a programming language you can pick up in a few minutes, especially if you have done any programming before.

Best of all, they are fun to use.

 

[dlgoff]

You could just purchase a http://www.martelmeters.com/products.php?cat=2" to measure the voltage across your 200kohm potentiometer. And you can use an op-amp between your pot and this meter to get the scaling you want.

 

[daveroderick]

I did look into a digital voltmeter last week for the system. It was pointed out to me that when I attempt to calibrate the timer I would be sending less or more voltage to the panel mount potentiometer there by changing the voltage coming out. So, if I was originally dispensing 12 ounces of liquid at say 3 volts, and I needed to calibrate the unit due to efficiency loss in the pump, when I adjust the calibration pot I would no longer get the same 3 volt reading for 12 ounces because I changed the voltage entering the potentiometer. When I was told this it made sense to me at the time.

The Picaxe chip sounds interesting and I will investigate them Monday when I am off work. You stated that the pulses on the chips never change which definitely sounds to be superior to the 555 timers. This however does not change the fact that I will have to calibrate the unit at some point due to changes in pump efficiency, rerouting of tubing and some other factors. I have used PLC controlled fillers that timed the pump to the millisecond and they had to be calibrated from time to time. I once had a system that used a 555 timer that had a vernier on the 10 turn pot, which was accurate. I guess what I was originally thinking of was a “digital vernier” for lack of a better term. Is there such a thing or can one be constructed?

Thanks,
Dave

 

[dlgoff]

I did look into a digital voltmeter last week for the system. It was pointed out to me that when I attempt to calibrate the timer I would be sending less or more voltage to the panel mount potentiometer there by changing the voltage coming out. So, if I was originally dispensing 12 ounces of liquid at say 3 volts, and I needed to calibrate the unit due to efficiency loss in the pump, when I adjust the calibration pot I would no longer get the same 3 volt reading for 12 ounces because I changed the voltage entering the potentiometer. When I was told this it made sense to me at the time.

That's why you would have an op-amp circuit. It calibrates the display not the pulse length.

 

[Bob S]

You referred to "timing the pump to the millisecond." Why don't you use the 555 timer output to gate a 1 kHz oscillator (using another 555 chip), and use a digital decade counter as your display.

 

[mellow]

I know, where it is possible to buy inexpensive in comparison with others LED DISPLAY .