Help understanding Circuit... Door Bell Controller - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: October 31, 2017, 08:04:56 am
content: I am trying to understand a 8-note Westminster chime controller (shown on a YT video, the images below are from a screen capture)...The 8 note sequence is created using a decade counter which provides 10 states which are N1 N2 N3 N4 P N5 N6 N7 N8 F where N is a note played by activating the solenoid and P is a pause. The Westminster chime is a sequence of four notes played once in one order and then again in a different order. The decade counter output combined with a few diodes creates the correct output order. P is a pause, F is the final state (waiting for next door buton press). Each note and pause is of equal duration. I understand that bit OK.However, the decade counter is such that the output on each line is ON for the full duration of that state whereas the actual requirement is to quickly hit the chime and then wait until the next transition. There is a HEF40106BP which is a hex Schmidt inverter... I imagine this is configured as a set of four one shots, one for each chime.I did some googling but could not find what I needed... basically is there a simple configuration of a Schmidt inverter (presumeably using RC) that would produce a short pulse from a long input pulse? Please can I have some clues? Thanks in advance

author: Benta
author_rank: Super Contributor
timestamp: October 31, 2017, 09:16:55 am
content: Sure, series cap to the input of the Schmitt trigger with associated pulldown resistor. Normally also a diode in parallel to the resistor to avoid negative input excursion.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: October 31, 2017, 10:57:12 am
content: Thanks (that was very informative)So the RC at the front of the inverter delays the output until the threshold is reached.... that's good. But the pulse doesn't actually shorten since the RC holds it until after the input the signal has gone (and then decays to turn off)? So it delays the incoming signal but doesn't shorten it? So to get a shorter pulse ending at the same time it would need to AND with the incoming signal?

author: Benta
author_rank: Super Contributor
timestamp: October 31, 2017, 04:02:35 pm
content: No.The cap is inserieswith the input, so when you input a low-to-high transition, the 40106 sees a positive spike on its input, which will decay as the resistor discharges the cap.The following high-to-low transition will of course result in a negative spike, which is why you need the diode to clamp this.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: October 31, 2017, 04:29:17 pm
content: Ah. I must have had that back to front...so... C then a D&R to ground in front...So when input goes Hi, big current, big potential difference across R, inverter fires, RC charges, current decreases and inverter flips when T- reachedThanks again.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: October 31, 2017, 11:50:13 pm
content: Managed to model it in Tina TI.

author: Zero999
author_rank: Super Contributor
timestamp: November 01, 2017, 09:42:49 am
content: Interesting circuit.How old is it?It looks like a really old design. Most musical door bells use an IC, such as the UM66. Although it's usually chip on board, so the part number is not visible.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 01, 2017, 01:38:02 pm
content: Quote from: Hero999 on November 01, 2017, 09:42:49 amNot sure how old... I don't own it... just saw the video on YouTube. It bangs chimes with solenoids to make sounds rather than being synthesized. I thought it was rather fun.