Part review – 4541 CMOS programmable timer

Hello readers!

Today we are going to examine the 4541 CMOS programmable timer IC. The main function of this chip is to act as a monostable timer. You are probably thinking one of two things – “what is a monostable timer?” or “why didn’t he use a 555 timer instead?”. A monostable timer is a timer that once activated sets an output high for a specified period of time, then stops waiting to be told to start again.  If you are not up to speed on the 555, have a look at my extensive review.

Although the 555 is cheap, easy to use and makes a popular timer, I have found that trying to get an exact time interval out of it somewhat difficult due to capacitor tolerance, so after some poking around found this IC and thought “Hmm – what have we here?”. So as always, let’s say hello:


As you can see this is a 14-pin package by Texas Instruments. It is also available in various surface-mount options. It is also currently available from FairchildNXP, ON Semi, and ST Micro. Note that this is a CMOS semiconductor, and that you should practice good anti-static precautions when handling it. Futhermore, when designing it into your circuit, don’t leave any pins floating – that is not connected to +5V or ground; unless specified by the data sheet. Here is the data sheet from ON Semiconductor.

This IC is interesting in that it contains a timer that can count to one of four values: 2^8, 2^10, 2^13, and 2^16. That is: 256, 1024, 8192 and 65536. With wiring you select which value to count to, and also the action to take whilst counting and once finished. This is quite easy, by connecting various pins to either GND or +5V. The following table from the data sheet details this:


And here are the pinouts:

The speed of the counting (the frequency) is determined by a simple RC circuit. For more information on RC circuits, please visit this post. You can calculate the frequency using the following formula:

There are two external resistors used in the circuit – Rtc and Rs. Rs needs to be as close as possible to twice the value of Rtc. Try and use 1% tolerance metal-film resistors for accuracy, and a small value capacitor. Also remember to take note of the restrictions printed next to the formula above.

Before examining a demonstration circuit, I would like to show you how to calculate your timing duration. As you can see from the formula above, calculating the frequency is easy enough. Once you have a value for f, (the number of counts per second) divide this into the count value less one power you have wired the chip. That is, if you have wired the chip up for 2^16, divide your frequency into 2^15.

For example, my demonstration circuit has Rtc as 10k ohm, Ctc as 10 nF, and Rs as 20k ohm; and the chip is wired for 2^16 count. Remember to convert your values back to base units. So resistance in ohms, and capacitance in farads. Remember that 1 microfarad is 1×10-6 farads. So my frequency is:


So my timing duration will be 2^15 divided by 4347.826 Hz (result from above) which is  7.536 seconds give or take a fraction of a second. To make these calculations easier, there is a spreadsheet you can download here. For example:


Here is my demonstration monstable circuit. Once the power has been turned on the counter starts, and once finished the LED is lit. Or if the circuit already has power, the reset button SW1 is pressed to start counting. You can see that pins 12 and 13 are high to enable counting to 2^16; pin 6 is low unless the button is pressed; and pin 9 is low which keeps the LED off while counting.


And my demonstration laid out (I really do make everything I write about):


 Easily done. Although this IC has been around for a long time, and many other products have superseded it, the 4541 can still be quite useful. For example, an Arduino system might need to trigger a motor, light, or something to runfor a period of time whilst doing something else. Unfortunately (thankfully?) Arduino cannot multi-task sketches, so this is where the 4541 can be useful. You only need to use a digitalWrite() to send a pulse to pin 6 of your timer circuit, and then the sketch can carry on, while the timer does its job and turns something on or off for a specified period of time.

Well I hope you found this part review interesting, and helped you think of something new to make. And if you made it this far – check out my new book “Arduino Workshop” from No Starch Press.

In the meanwhile have fun and keep checking into Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

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John Boxall

Founder, owner and managing editor of

6 Responses to “Part review – 4541 CMOS programmable timer”

  1. Mitch Davis says:

    Nce artcle, thank you. Not sure I see the pont of usng 1% resstors f the capactor that goes wth t s, at best, 5% tolerance.

    • John Boxall says:

      Hello, thanks for leavng your thoughts. Yes, true – no real need for 1%. It’s more of a personal habt to specfy them, and ths came through n the artcle.

      • Mitch Davis says:

        But thank you for wrtng the artcle, please wrte more!

        For me, by the tme t would take me to fnd a 4541, and desgn t nto the crcut, and debug t, then wsh I hadn’t used hardware when t doesn’t work qute rght n the end product, I start wshng I’d worked out how to get some small smultaneous acton gong n the Arduno.

        Yes, you can’t multplex sketches, but you can stll fddle around wth the tmers and have nterrupts trgger the callng of solated routnes. The advantage of dong that s your behavour s defned n software, and, to wthn a few cycles, s accurate.

        Another dea s to look at a small tmeslcer for the Arduno. The “Ako” framework has one that’s easy to use.

      • John Boxall says:

        H Mtch
        Thanks for your feedback. You’re rght about usng t wth Arduno. Lately I have been lookng nto dong thngs the 20th century way, whch explans some of these part revews. Wll have a look nto the Ako group as well.

  2. David Seal says:

    Love the artcle. Very helpful. There are other reasons to use the 4541, though. Number 1: t s far cheaper than a PIC, both n actual costs, tme and support components. 2: It uses very lttle power compared to most PIC’s. Ths s very mportant f t s battery powered, and/or f the tme out nterval s on the order of hours. 3. It operates fne, wth very lttle drft, rght down to 3 volts and below, dependng on the make. Agan, mportant n applcatons where the voltage avalable may fluctuate, no need for power-hungery voltage regulator..

  3. Dario says:

    John, would lke to make a queston a lttle bt out of ths post topc. Ths s the thng, ‘ve been dong a source of alarm clock wth an arduno (UNO) and a DS1307, and to generate the alarm nose was usng an ne555 and a buzzer, the pont s that ths ntroduces too much nose, so wonder f use ths c t wll ntroduce less nose that the ne555?


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