C1 is a noise suppression capacitor that actually plays no role in the timing. R1 is a pullup resistor to hold the reset pin and the trigger pin high until SW1 is pushed. The timing is controlled by C2 and the choice of R2, R3, R4, or R5.

In operation, the user operates a switch to select either R2, R3, R4, or R5 depending upon how long a delay desired, and then presses SW1 to start the timer. If you would be satisfied with that operation, then this would be a solution for you. I will be happy to calcutate the appropriate values for the resistors and capacitor if you want to use this.

If you must have a separate pushbutton for each of the times you want, then I think that a separate 555 circuit for each time is the simplest choice. That would mean four similar 555 circuits, one for each of the four times you want. If that is what you want, I will draw a diagram and post it.

Just say which solution you want and the times. Don't forget that the times won't be exact because of the tolerance in components and the effects of temperature and aging.

thx..hmm, can you plz draw the circuit for both esp the one with 4 555 timers.
For the timing, since I'm nt sure why 470µF is used. So i continue using the same value to obtain the timing. (Or maybe I shld chg the value of capacitor?)

10 mins: 1.1 x 1.1MΩ x 470µF = 568 secs ≃ 9.48 mins
15 mins: 1.1 x 1.6MΩ x 470µF = 827.2 secs ≃ 13.78 mins
20 mins: 1.1 x 2.2MΩ x 470µF = 1137.4 secs ≃ 18.96 mins
30 mins: 1.1 x 3.3MΩ x 470µF = 1706.1 secs ≃ 28.44 mins

What do you think?

 

I would try to keep R's to no more than 1 meg, or 1 meg pots in series with 100k, and vary C's. C's around 2000μF.

 

Attached is my schematic; it turned out to be a little more complicated than I thought, but then doesn't everything?

There are four almost identical timer circuits, each triggered by a pushbutton. I used 1000 μF caps in all four: one each in the two shorter times and two each in the two longer times. Since you know the formula, you can check my math and adjust the times as you see fit. As Bernard suggested, you might want to decrease the R values a little, and add a pot to fine tune the times.

I am fairly confident in the first four timer circuits, but after I drew them, it occurred to me that a fifth timer was needed to control the beeper, so I added it. Here's the way I think it should work, but I haven't built it, so I am not sure.

When any one of the four pushbuttons are pressed, the corresponding pin 3 will go high and, and via the diode OR matrix, will take the trigger pin on U5 high. When the time expires, the trigger pin of U5 will go low, thus triggering U5, and its pin 3 will go high and stay high for 1 second, thus sounding the beeper. Bear in mind that only DC is coming out of pin 3, so you will need a beeper there, not just a speaker.

Because I haven't built this, I hope Bernard and/or others will do a check on the schematic and make corrections as required.

Thanks.

ETA: There are simpler ways to do this using a microcontroller, but that requires programming skills and tools.

 

OK, but the OP didn't want a switch; she wanted 3 different pushbuttons, each of which would be for a different delay time. That's what caused me to suggest three 555 circuits instead of one.

Thanks Tracecom, I had assumed the "3 buttons" concept was a naive design goal and that duirng implementation it would get fudged to a more functional design like a rotary switch to set the time and a button to start it. I have a commercially made kitchen timer that uses a rotary switch and start button, all for the same reasons. Its start button is actually a push on/off latching type with vertical travel (so you can easily feel if on/off) which solves other problems.

How does your circuit cope with user fault conditions ie the visually impaired person presses two buttons at the same time?

Really, the best implementation would be a 50 cent 8pin microcontroller and about 15 lines of code.

 

How does your circuit cope with user fault conditions ie the visually impaired person presses two buttons at the same time?

Really, the best implementation would be a 50 cent 8pin microcontroller and about 15 lines of code.

If it works like I think (which is a big if), the longer time will take precedence.

Yes, the best choice would be a microcontroller. But that is the case more and more often, which is the reason that I am once again trying to learn PICs as opposed to studying analog concepts.

And if I seemed abrupt with my response, it was unintended. I have great respect for your opinion.

 

With all 4 relativley long times, no need for switch bounce elimination, so might eleminate cap coupling on inputs. On powerup, U5 is going to trigger, maybe no problem, but OP wanted beep to last untill power off. I was going to make U5 a SCR or long time 555. But it is late.

 

With all 4 relativley long times, no need for switch bounce elimination, so might eleminate cap coupling on inputs. On powerup, U5 is going to trigger, maybe no problem, but OP wanted beep to last untill power off. I was going to make U5 a SCR or long time 555. But it is late.

You are correct on both counts. I have removed the signal conditioner from all four switches, and have deleted U5 and associated circuitry. I don't have any SCR experience or parts. Would you advise the OP (and me) of how to connect the diode OR output to the beeper? Thanks.

 

The 555's will trigger on power-up if reset is not held low for split second, at least that what I see on bread bd. Solved with 100k & 1 μF on pin 4. I believe all pin 4's could be tied together with one delay ckt. U5 might stay if time stretched, 10min max?? The SCR version is sensitave gate 2N5060, 30V, .5A. Transistor is 2N3904. Pins down flat forward, left E, B, C; K, G, A. Missing R value on base 20k.

 

The 555's will trigger on power-up if reset is not held low for split second, at least that what I see on bread bd. Solved with 100k & 1 μF on pin 4. I believe all pin 4's could be tied together with one delay ckt. U5 might stay if time stretched, 10min max?? The SCR version is sensitave gate 2N5060, 30V, .5A. Transistor is 2N3904. Pins down flat forward, left E, B, C; K, G, A. Missing R value on base 20k.

Thanks Bernard. Here's the revised schematic. I made the reset pin cap 1 μF as you said in the text, and the Q1 base to ground resistor 22k to get to a standard value. Let me know if there's anything I need to change.

 

Nice job, tracecom & fast. Just have to nit pick- R4 no value is 100k. First I was worried that U's would not stay reset @ turn on, now 'wonder if SCR will behave @ turn on. Could add RC delay above R12, something like 1k @ 10μF, 'will check it out.
Found 2N5060's, & tried it out- yes it triggers on power-up; added 1k in series with R12 & +9V [ OP said she??, was using 9V battery ] with 10 uF junctoion of R's to gnd. Walla-no falsies. Changed R13 to 3k.
I have a few 2N5060's available, left overs from a 1970's alarm project.

 

A little side project: To check large C values, I set up a 555 as one shot with 240k, 1 % timing resistor & semi unknown value of C. Used a LED on output & timed with stop watch. Using a 2200μF cap, first checked for leakage, then checked time, 10m 34s; calculated value of 2400 μF- not bad, about + 9%. With close C values, should hit desired timing fairly close with fixed R's- no pots.

 

Nice job, tracecom & fast. Just have to nit pick- R4 no value is 100k. First I was worried that U's would not stay reset @ turn on, now 'wonder if SCR will behave @ turn on. Could add RC delay above R12, something like 1k @ 10μF, 'will check it out.
Found 2N5060's, & tried it out- yes it triggers on power-up; added 1k in series with R12 & +9V [ OP said she??, was using 9V battery ] with 10 uF junctoion of R's to gnd. Walla-no falsies. Changed R13 to 3k.
I have a few 2N5060's available, left overs from a 1970's alarm project.

OK, I have attached the latest version. Thanks again.


26 March 2013 ETA: Deleted schematic; see post 51 for final schematic.

 

Looks like a winner.

 

thx tracecom & bernard..I will try to do it & gv feedbacks here..tq tq tq^^.

 

Sorry I'm here again. Is it possible to carry out my project by using a microP? I intend to do both ( one with the ne555 quad monostable & one with microP)..then i compare the results.

 

Sorry I'm here again. Is it possible to carry out my project by using a microP? I intend to do both ( one with the ne555 quad monostable & one with microP)..then i compare the results.

Yes, it is possible. What kind of microcontroller do you plan to use? PICAXE is the easiest and least expensive to start with. An unverified schematic is attached; of course, you would also need code to make it run.

The AXE021 includes all the hardware you need except the 4 switches, 8 resistors, the PICAXE 08M2 chip, and a power supply (5V).

http://www.picaxe.com/docs/axe021.pdf

 

Yes, it is possible. What kind of microcontroller do you plan to use? PICAXE is the easiest and least expensive to start with. An unverified schematic is attached; of course, you would also need code to make it run.

The AXE021 includes all the hardware you need except the 4 switches, 8 resistors, the PICAXE 08M2 chip, and a power supply (5V).

http://www.picaxe.com/docs/axe021.pdf

Sorry, what is AXE021? I plan to use PIC16F877A which is available in the lab. For the programming part, I will be using MPLAB. And one more ques, what is the diff for using 5V & 9V?

 

The AXE 021 cable is for PICAXE chips.

Picaxe Home

They have their own Pre-Masked PICs with the Bootloader and BASIC engine installed on the IC.

You can't turn a PIC into a PICAXE. You don't need MPLAB or a PICKit.

I suggest this one The price is right. The PICAXE programming editor and manuals are a free download from the link above.

You can write your timer program, simulate it visually, and debug it with no hardware present. There may be some slight port issues once you get the hardware, but overall, it's an easy to use and perfect system for projects like this one.

Add in the Real Time Clock Module (I²C) or build your own for very accurate long delays (See stop light timer thread above for a huge head start).

If you already know how PIC ports function, and any programming language, you should have no problem getting one up and running in no time. This could even have provisions for adjustable time, 3 set times, 3 concurrent timers, abort buttons, etc. It could be made simpler as well.

Benefits: Minimal components. 3AA batteries, 3 buttons, 1 piezo alarm, that starter kit and clock module (you could get "close enough" timing with onboard oscillator) and some bypass caps for debouncing the switches and to give the uC a clean power supply.

 

The AXE 021 cable is for PICAXE chips.

Actually, the AXE021 is a prototyping board for the PICAXE 08 chip.

 

Actually, the AXE021 is a prototyping board for the PICAXE 08 chip.

Sorry about that. I wish PICAXE used the 100 series for boards, 200 for programmers or something like that. Maybe they plan for a lot of expansion?