Trying to reverse engineer a clock with chime: Boost - Page 1
Forum section: electronics / beginners

author: NivagSwerdna
author_rank: Super Contributor
timestamp: August 24, 2021, 10:31:00 am
content: I'm trying to reverse engineer my chiming alarm clock... the squares are connection points to the uP board...I would expect the uP to provide the oscillation to the boost but probing the signals seems to suggest no such oscillation so I'm wondering if this circuit self-oscillates and sadly I don't quite understand how it works... suggestions appreciated.I think the Q3 and Q4 pair is for discharge.Q1 and Q2 are something to do with the boost... I think there is an small inductor to the positive supply (3V).I'm guessing the connection at the top is for sensing the voltage of the charged capacitor but no idea why there is a diode.I think the connection at the top is to control either high or low mode... the diode is a zener and when connected to ground it limits the boost voltage... possibly...Please could someone suggest how this would boost.Thanks in advance

author: NivagSwerdna
author_rank: Super Contributor
timestamp: August 24, 2021, 06:04:49 pm
content: I think one or more of the diodes might actually be zeners

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 17, 2021, 02:29:45 pm
content: Here's my current version of the reversed schematic...

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 17, 2021, 02:35:19 pm
content: I don't quite understand it...Q1 and Q2 looks like the switching for the boost side. A square wave applied to J22 should make the boost work although I don't quite understand the C4/R2/D4 arrangement. (The actual PCB uses 9014C transistors)The Q3 and Q4 arrangement likewise appears slightly odd; I would guess that a HI to J23 turns on the pair to make the C5 dump through the solenoid L2. Looks like an upside down darlington pair?J24 perhaps is for 'low power mode'... when Grounded it would cause D6 to limit C5 to 3V9? No resistor?Not sure if my understanding is dodgy. Any insight appreciated.

author: magic
author_rank: Super Contributor
timestamp: September 17, 2021, 06:22:55 pm
content: The version in the first post could perhaps self-oscillate when small current is provided through a pullup resistor from the logic board.The idea is that when inductor current reaches β·Ibase then voltage at the collector starts to rise and turns on Q1 by means of the capacitor, and Q1 briefly turns off Q2.Q3,Q4 is an ordinary Darlington pair. Not sure what's the deal with 100Ω.The version you posted today can't work, there is nowhere for Q2 base current to come from. The freewheeling diode at the solenoid should also be connected as in the first post, unless it is not actually a freewheeling diode.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 17, 2021, 07:44:31 pm
content: Checked again (although it is a bit hard to decode the PCB)... The diode was wrong... and J22 was incorrect.Revised.

author: magic
author_rank: Super Contributor
timestamp: September 17, 2021, 08:20:40 pm
content: Yes, it looks like a self-oscillating boost, as described above. Similar to the popular ringing choke converter, used inside all the dogiest-ass wall warts.I think R2 is wrong, it made more sense as drawn originally.There used to be a very good post on some blog somewhere explaining the RCC with all its common variants, but I can't find it now

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 17, 2021, 08:34:57 pm
content: I did some measuring... I see J22 go to 3V for the 5 seconds before it is meant to chime.Looking at the clock itself it appears that J22 is driven by the uP with 3V through a 1k resistor when boost is ON. (disconnected trace shown first)I cannot quite see how that combo would oscillate so any pointers to resources would be gratefully received.If I probe the J22 connection at the boost converter then I see something funky going on...

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 17, 2021, 08:40:25 pm
content: I think I'm starting to understand your Reply#4! Thanks!

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 17, 2021, 11:11:57 pm
content: R2 moved to correct place

author: magic
author_rank: Super Contributor
timestamp: September 18, 2021, 06:36:25 am
content: Q2 is turned on through J22.C4 discharges through Q2 and D4, Q1 sees between 0V and -0.7V at its base.L1 sees 3V and its current increases linearly, later it may even saturate and its current jump to the sky.When L1 current is too high for Q2 to sink, Q2 collector voltage starts to increase.C4 charges up; the charging current can't flow through D4 so it flows through Q1.Q1 shuts down Q2, Q2 collector voltage raises even faster, Q1 is turned on harder.L1 dumps its current through D5 into C5, a high voltage is developed.C4 negative pole is now at several volts, R2 slowly pulls current through C4 and charges it up, Q1 is kept active.L1 energy is discharged, voltage drops down, C4 drops down and turns off Q1.The cycle restarts.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: September 18, 2021, 07:49:00 am
content: Brilliant! Thanks for taking the time to explain that, it was beyond me!