Reverse Engineering a 555 Ding Dong Door Bell Kit - Page 1
Forum section: electronics / beginners

author: jonovid
author_rank: Super Contributor
timestamp: July 30, 2020, 07:51:07 pm
content: here is a $1 Chinese electronics kit, without an instruction manual or schematic diagram.that was optimised to be dirt cheap by using the same components multiple times.see the 47k and 103 10nf used multiple times in the same circuit. see imagesthis circuit is barely working, but it does work. I did some reverse engineering, now I am showing you the circuit.i think the circuit needs more caps on the power bus and a NPN to do or help the switching the 4 pinalso the video shows high current drain with the 555 getting hot . see video at 10:46I have not reproduced the circuit to mod or fix it only done image & artwork.see images and judge for yourself , if this is cutting it a bit too much, to save on components.eBay Cheap_ $0.75 Electronic Doorbell Kit NE555__360p

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 31, 2020, 01:59:24 am
content: You're traced schematic has an obvious error and thus will not oscillate.The reason the kit won't work at the beginning is due to the push button. The quality of these are exceptionally bad and would not make good contact if not being used frequently. Although the video author said he hasn't done anything to the kit, this is not strictly true. His repeatedly pressing the push button somehow overcome the contact resistance and enable the push button to work again.In general trouble shooting, always check that these button shows contact resistance below a few ohms when depressed.

author: floobydust
author_rank: Super Contributor
timestamp: July 31, 2020, 05:08:27 am
content: I used pin 5 Control Voltage for the bell sound, but this works too. I think it would eat batteries with the555 ICpowered up all the time, maybe 5-10mA. So I would say it's tooooo cheap.OP in your schematic the resistor at pin 7 connection not right. This schematic I found might be the same circuit.

author: jonovid
author_rank: Super Contributor
timestamp: July 31, 2020, 04:12:39 pm
content: Quote from: floobydust on July 31, 2020, 05:08:27 amyour right about current drain, I would re-design the circuit so its open circuit when not in use.with a electrolytic capacitor charge & discharge powering the 555.my interest in the pcb circuit in the YT video was because of the use of the 47k and 103 10nf used multiple times in the same circuit.when maybe better component values could be used.

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2020, 10:25:48 pm
content: I can't see an easy way to redesign it so the 555 isn't powered, when not in use. If the power is disconnected, when the switch is released, then it will stop producing the tone.I wonder what impedance that speaker is? It's possible it's too low for the 555 timer to drive, at 9V, which is why it gets hot. This probably isn't an issue, given the low duty cycle.The NE555 could be replaced with the low power ICM7555, but it won't be able to drive the speaker properly, so booster transistor(s) would be required.

author: jonovid
author_rank: Super Contributor
timestamp: August 01, 2020, 08:42:25 pm
content: Quote from: Zero999 on July 31, 2020, 10:25:48 pmyour right about this circuitif the goal of this circuit is to keep it as cheap as possible ,however it fails as a functioning door ball kitas it requires the cost of a plug pack or power adapter to be a functional door ball,a D.I.Y. electronic toy, this circuit works well.goal:make a new circuit using a 555 or ICM7555 or other ic and the minimum of other components.must run from a common cheap battery type , a 9 volt battery so no current draw when in standby or off.around 100 mA maximum for 0.5 to 1 second Push to make button power on & power up a electrolytic,the ding. 0.4 of a second to power up= make a sound,plus power the 0.5 second dong sound when the button is released.a pin 4 on & cut off threshold or envelope generator so the circuit will not sound like a dying catif and when. then design a new pcb for it , or........... need more time to think about this,as 9 volt battery is not so cheap need totest new ideas,