Simple temporary soft-on circuit request - Page 1
Forum section: electronics / beginners

author: brainwash
author_rank: Frequent Contributor
timestamp: March 13, 2013, 07:54:00 am
content: I want to make the simplest possible circuit that accomplishes this: upon applying 220V at one input it drives a uC pin (3.3V) to ground for 500ms.Some background: I have connected a coffee machine (Philips/Senseo HDxxxx) to a remote outlet and the original design is that you have to press the soft power-on button to get it to heat, just plugging in the machine doesn't do anything.It could be a relay that is NC and it stays closed for 500ms after power is applied, then opens. I would like to avoid a relay if possible, I don't have a lot of space inside the unit.I could just throw in a uC but that seems overkill.I don't know, maybe some voltage divider feeding a simple rectifier that drives a an optocoupler until some capacitor charges up. The on-time should be not more than 2 seconds and probably not less than 50ms.

author: brainwash
author_rank: Frequent Contributor
timestamp: March 13, 2013, 10:46:28 am
content: I over-complicated the requirements a bit, as far as I remember 3.3V is always on.Attached is a simplified schematic that grounds the switch input 200ms after power-on.Some explanations for beginners attempting to build something similar:Since the machine uses the HT46R47 MCU (http://www.gaw.ru/pdf/Holtek/uc/46r47.pdf) Philips probably saved cost and used the internal "pull-high" option instead of an external pull-up resistor.The MCU datasheet shows the part is able to function from 3.3V to 5V and the pull-up values for that voltages are respectively 60k and 30k. Circuit function is independent of pull-up value, if above 10k.If the chip is indeed powered up at 5V then the on-time of my circuit is reduced to ~140ms but should still be enough. Human microswitch press time is usually around 200ms. If a longer delay is needed then C1 or R2 can be increased.Any generic npn transistor can be used.

author: micro_freak
author_rank: Contributor
timestamp: March 15, 2013, 01:13:33 am
content: Not sure I fully understand your requirement, but maybe a good old 555 in monostable configuration would do the trick.

author: Christe4nM
author_rank: Unknown
timestamp: March 16, 2013, 09:56:33 am
content: I suppose a plain simple R-C circuit followed by a comparator should do the trick as well. You set the time constant for the comparator as required. You'd have to make sure the C is 'empty' upon power-on though.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 16, 2013, 10:47:52 am
content: I presume that what actually happens is when you plug in the 220V, the 3.3V rail powering the microcontroller comes up, but the microcontroller doesn't activate the heater straight away - correct? In that case, you don't want or need anything which actually connects to the mains, it just needs to be activated when the 3.3V dc supply comes up. Am I right?If so, then the part you need is an On Semiconductor NCP302 or NCP303. This tiny little device is designed for exactly this purpose - ground a pin for a period of time when power is applied, then either release it and let an external resistor pull it high (303) or drive it high (302).One capacitor sets the time delay, and you won't find anything smaller or simpler to use.http://www.onsemi.com/PowerSolutions/product.do?id=NCP303

author: brainwash
author_rank: Frequent Contributor
timestamp: March 16, 2013, 12:54:57 pm
content: I've already posted the solution in the second post, is as simple as it gets.Unfortunately I did not use this circuit because I needed to use a uC anyway.