Countdown Timer (Buzzer Triggering)

Thread Starter

Pete D.

Joined Jun 3, 2015
Hello fellow members.

I'm new to the AAC community and, as a budding electronics hobbyist, I could surely benefit from
those of you who have greater expertise. If anyone can provide some assistance regarding my dilemma
noted below, I'd be greatly appreciative.

Project: Presettable Countdown Timer that stops at zero and sets off a buzzer.


1. Attached circuit is a working draft created in Circuit Wizard simulator.
2. Circuit will later be expanded from 2 digits to 6 digits.
3. BCD Thumbwheel input to presettable 4510 counter temporarily omitted from design.
4. LED temporarily substituted for alarm buzzer.

Technical Overview:

Astable NE555 Timer set for 1 sec clock pulse (1Hz).
Presettable BCD input of 4510 is temporarily set to 20 sec - (L2) set High on Display 1 (DS1)
SPST Momentary (SW1) Loads presettable BCD and triggers astable countdown.
SPST Momentary (SW2) used for reset.
Buzzer 1 tied to both switches (SW1, SW2) for audio feedback when pressed.
OR Gate 4071 is tied to Carry Out of both 4510 counters and stops clock at zero.
Monostable NE55 Timer for Alarm portion of circuit set for 4.7 sec duration.


1. SPST latching switch for clock trigger avoided intentionally for design / functional purposes.
2. NPN Transistor (Q3) intentionally left floating pending triggering mechanism.

Problem / Dilemma:

I'm unable to figure out how to trigger the monostable alarm feature via NPN Transistor (Q3) when
the countdown timer stops at zero. I struggled with the Schmitt Trigger, 4093 NAND, and other logic
gates without success.

Note: Both the astable timer circuit and the monostable alarm portion of the circuit function
properly independently.

Please assist with any advice or direction regarding a solution.

Thank you kindly!



Joined Aug 1, 2013
Switching power to a 555 is not the way to trigger it as a monostable. Thi is covered in the 555 datasheet. Use the carry out signal from the lsb counter to trigger the 555.

Improve your schematic by making sure that every input pin on every part is accounted for by connecting it to a signal, Vcc, or GND. Also, add pin numbers to the ics to make it easier for people (and you) tosee what you are trying to do.


Thread Starter

Pete D.

Joined Jun 3, 2015
AnalogKid, thank you for your speedy response and direction. I will work on redesigning the one-shot monostable and try to trigger it with the signal output of the counter as recommended. I'll update the thread accordingly upon completion.

Thanks again for your time and patience. Much appreciated!


Joined Jan 2, 2010
Hello Pete D.

There are some things to improve in the circuit shown in the document that you enclose in your original message:

The U/D inputs(10) of the IC's 4511B must be connected to ground to count downward.

Input EN(5) of IC4 must be connected to ground so that this counter is enabled.
Therefore, as well as looking at the scheme, it will not count when building the circuit in reality.

ST(5) inputs of the IC's 4511B must be connected to ground so that input Strobe is false.

It is best to use buttons normally closed, as in this case, and Pull-Up 10K resistor. See attachment.

I guess the BZ1 is to do a 'click' when any button is pressed, whether we call the Load or Reset button.

To the alarm when the counter reaches zero: I recommend using a Flip-Flip SR.
When the counter reaches zero put in Set the Flip-Flip. And With Its Q enables 555 (PIN 4).
Not Shown in my attached files.

To turn off the alarm use another button connected to the Clock(3) Flip-Flip.

I hope this will help you continue with your project.


Thread Starter

Pete D.

Joined Jun 3, 2015
Hi MrCarlos, please pardon my delayed response as I was out of town last week.

Thank you kindly for the helpful information you provided along with the Circuit Wizard sample of the SR Flip Flop. Your assistance is greatly appreciated!

I revised my schematic to include many of the recommendations you mentioned and attached the associated pdf file along with a working .cwz for your review. I beefed up my schematic to include additional 7 segment displays for minutes/seconds, gate control, and the alarm buzzer. I opted to remove the feedback buzzer for the Load and Reset push buttons (NC) as this was overkill.

Please be aware that I left Pin-4 of IC10 (Monostable) unassigned as I ran into a few undesirable problems while testing the SR Flip Flop solution. Note: The SRFF was omitted as well.

Open Issues:

1. SRFF worked fine to stop the clock (Pin-3) on the Astable 555 and trigger the alarm when count reached 00. However, since the SRFF is of the latching type the monostable would continue to remain High (1) preventing re-triggering for a subsequent BCD load. This may be more of a function of the self-triggering/one-shot monostable?

2. SRFF stops the clock on Pin-3, but when I activate the next BCD load on the counter, the subsequent load value on the 7 segment display sometimes blinks super-fast. Example: 01:00 goes to 00:59 in seemingly a nano second. This is probably because the button press is catching the positive edge of the clock pulse. I don't experience this when I stop the clock by bringing Pin-4 (Reset) low in my current circuit design. I'm hoping to keep it this way.

3. The Reset Button (SW2) when actuated during the countdown process, resets each counter's carry-out value LOW (0), the same Logic Gate values that are used to trigger the SRFF and stop the clock. Hence, by pressing the Reset Button, the alarm can be triggered separate from the countdown method. This is undesirable and poses a problem whether using the SRFF or an alternate trigger.

Present Objective(s):

1. Keep the main Countdown timer circuit (astable) relatively as is because it's working perfectly.
2. Figure out an alternate solution for triggering, and subsequently re-triggering, the monostable for multiple BCD load events.
3. Prevent the Reset Switch from triggering the monostable alarm.

I hope the information I provided makes sense as I'm new to electronics.

If you have additional recommendations please share.

Thanks again!



Joined Jan 2, 2010
Hello Pete D.

Your design looks fine. But, please, make this experiment:
Run the simulation.
Press the shift key in you computer Keyboard.
At the same time push the Load Button With The Mouse.
Release every thing.
The load Push button must continue open.
One by one: program some time with the electronic switches; You Will See that IC6 do not change, why?. . . Because this counter is not well connected.
By the way: You could not find these kind of electronic switch in the market.

Thread Starter

Pete D.

Joined Jun 3, 2015

Thank you for the feedback. The existing Load Switch (SW1) is a Push-To-Break switch that you suggested I use in your first communication to me. It's the same one you employ in the Flip Flop .cwz you shared earlier. Being that the switch is grounded and normally closed, no voltage from the 5V rail can go to the Load input (Pin-1) of the 4510 IC counter, hence it remains LOW (0). Only when SW1 is pressed will the load input (Pin-1) of the 4510 IC counter go HIGH (1) and register the BCD values set on input of the counter. Upon release of SW1 the IC14a IC (OR Gate), which is now High (1), triggers the countdown.

Holding the Load switch down in a latched fashion, as in the experiment you asked me to conduct, loaded the correct BCD values to the display and froze the countdown until my release of the Load Switch (SW1). I believe this is how the SW1 momentary switch is supposed to behave so as not to keep the input (Pin-1) of the counter HIGH (1).

Regarding the IC6 IC, I have intentionally kept it disconnected from the Load as I don't want the BCD 1001 (9) input I use to maintain a 59 sec loop, to be displayed during a load. Instead, when the Load button is pressed IC6 is triggered to Reset via the IC13a (OR Gate) so the display reads zero. Note: I am only planning to use two (2) BCD Thumbwheel inputs for the "Minutes" portion of the countdown timer (IC2, IC4). The "Seconds" portion (IC6, IC8) will remain static and not user defined.

Regarding the limited availability of the Push-To-Break switches in the market; I will likely revert back to Push-To-Make switches when I'm ready for production.

Right now I'm still struggling with the "Open Issues" noted in my prior thread. I will continue to work toward a resolution.

As always, thanks for your assistance!