Basic Announciator - Page 1
Forum section: electronics / projects

author: nailtherail
author_rank: Newbie
timestamp: April 17, 2023, 02:20:37 pm
content: I have a 12v circuit which is mainly logic based, with the odd switch, to create a desired output which lights a specified number on different LED seven segment displays.There is nothing to stop the push button shown being pushed multiple times, or even how long the push button is pressed each time. Each time the button is pressed, it restarts the timer to a 3 minute countdown.However, I am hoping to be able to add a basic audible sound (to announce the initial button press event occurring) of about one second via a bell or buzzer, which only sounds when the push button is INITIALLY pushed and then should not trigger again until the output of the 3 minute timer resets itself again (Back to 0v).I have included a basic schematic of the current circuits design with the purple showing possible sounder circuit path.Can anybody recommend the best way to achieve the above??It was my aim to get the sounder to possibly sound once if the switch was in position '1', twice in position '2' and three time in position '3' but I think this is probably too complicated!! (unless you know and can suggest a way otherwise)

author: Terry Bites
author_rank: Super Contributor
timestamp: April 17, 2023, 03:26:12 pm
content: Use a non-retriggerable monostable to lock out the trigger to the buzzer, a 555 (OMG not again) perhaps.The trigger will cause the mono to trigger the buzzer. The mono will ignore all further inputs untill it has timed out.

author: nailtherail
author_rank: Newbie
timestamp: April 17, 2023, 03:51:53 pm
content: Terry, I am fairly new to this, any chance of a circuit diagram. Thanks for your response.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 17, 2023, 04:12:59 pm
content: Luckily for you I couldn't get off to sleep last night.My understanding is that the LEDs will go dark after the timeout?There are simple low cost dedicated timer chips. Most common of which is the "555" and its variants.A "7555" will be needed to reliably obtain longer timer periods.Pushing PB starts the 7555 timer, it will ignore the PB untill it times out.At the end of 180s the LEDs go off. To add a fixed time for the buzzer, another timer will be needed.BTW you can get two 7555s in one package, the 7556.You have to ask yourself if the gates can drive enough current into the segments.The same applies to the buzzer, how much current will it need?Gates able to operate on 12V (CMOS "4000 series") can only supply a few mA max.Usually resistors are needed to set the LED current for a consistent brighness.Gates are more typically run on 5V so you'd use a voltage regulator to get you down from 12V. ("78L05")In a more real world, and somewhat more complex, solution you'd use transistors to switch the LEDs and Buzzer.AND gates lack a versitility and its more effective to use NAND and NOR gates to achieve the same logical results.To get all the functionality you'd like, a "state machine" is needed. This can be made from "sequential logic" which is both time consuming and complex.Alternatively you might use a microcontroller but that would be overkill and you'd learn little of use.Niether are really worth the effort they're not beginners projects.

author: nailtherail
author_rank: Newbie
timestamp: April 22, 2023, 10:24:43 am
content: Terry,Many thanks for your response and the circuit diagrams. Whilst I originally showed a very basic circuit showing the three switches and LED's as outputs, there are indeed transistors already on the outputs of the logic circuit/s to control the LED's, plus resistors for each. I also have resistors on each switch to stop floating spurious inputs into the logic but didn't think it necessary to show these on a vey basic circuit which was only to give people an idea what I was after.I have been previously tied to using AND, OR and NOT gates due to the previous design continuously evolving into what it is now or modules merely being added on.The entire system controls a model railway signalling system. and it is purely the add on of a sound warning that I wished to produce. In real terms, the switches are actually reed switches mounted under the tracks.