555 astable timer help - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: September 05, 2019, 11:44:58 pm
content: hi all,ive built an astable 555 timer with the intention of using the output pulses to switch a mosfet to drive a solenoid vave,thing is i didnt realise pin 3 output is a cuppent sink not a source,its opposit to what i expected,any idears of a way to overcome this?,its got me stumped,ps. the power budget is tight as its battery poweredi ie whatever i use must have a low quiessent current.,cheer Paul m3vuv.

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: September 06, 2019, 12:10:33 am
content: Quote from: m3vuv on September 05, 2019, 11:44:58 pmLow Iqis not the 555's specialty. An MCU would be much more effective.Pin 3 is push-pull. If your problem is that the sense of the output is inverted (active-low) from the sense that you want (active high), change your want and turn the MOSFET and solenoid segment upside down instead. Note the change to p-channel MOSFET, other circuitry and protection diodes not shown for clarity, drawing not to scale, etc. etc.

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2019, 07:42:33 am
content: Quote from: jhpadjustable on September 06, 2019, 12:10:33 amHow do you know that? There isn't enough information to determine whether a MCU would be a better choice, in this application. There are plenty of reasons to use a 555 timer, over a microcontroller. For example, if it's running off 12V, a 5V regulator and a logic level MOSFET would be required with an MCU, but not with the old 555 timer.Quote from: m3vuv on September 05, 2019, 11:44:58 pmA 555 timer can both sink, as well as source current. In fact it's capable of sinking/sourcing more current, than most microcontrollers, so I don't think that's the issue here.Please post a schematic.

author: Renate
author_rank: Super Contributor
timestamp: September 06, 2019, 09:34:27 pm
content: Ah, same project, different thread.A 555 with a standard two resistor astable can only deliver an output which is more of the time high than low.Pin 3 (totem pole output) and pin 7 (open collector) operate in the same direction all the time.Want independently adjustable on and off times?Use the pin 3 output through two steering diodes and two pots to the C/2/6.The P MOSFET is a good idea, as long as the solenoid supply is the same as the 555 supply.Of course, as a joke you could drive the P MOSFET with pin 7 and a pullup resistor.I'll bet you could run a lot higher voltage than the absolute maximum (18V) for the whole chip.You could power the 555 with 5V and maybe run 48V on pin7.Who wants to try?Of course a solenoid valve won't give you much accuracy on dispensing.I presume that you've already ruled out a peristaltic pump?

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2019, 10:08:06 pm
content: I don't know what the maximum voltage rating of the discharge transistor (pin 7) is. I doubt it's much more than the maximum rating of the IC. To use it at higher voltages, add another NPN transistor (base to +V, via a suitable resistor and emitter to pin 7) to form a cascode with the internal discharge transistor.

author: Renate
author_rank: Super Contributor
timestamp: September 07, 2019, 06:40:58 pm
content: Oh, I always liked cascode amplifiers.They were used quite often for driving cathodes on CRTs.The common base eliminates Miller effect allowing high frequency operation.But my point was a silly one:Using just a drawer of 555's you didn't like anymore, what is the highest voltage operation you could get on pin 7?

author: m3vuv
author_rank: Super Contributor
timestamp: September 09, 2019, 01:13:35 pm
content: as far as i can tell with 12v vcc ,ie working it from a 12v battery,pin 7 is outputting 5v when the led is off and 0v when its on,ie when pin 3 goes low so does pin 7,hope that makes sense,also now they want it to work for 12hrs then stop for 12 hrs,can this be done using a 556 ic?

author: Renate
author_rank: Super Contributor
timestamp: September 09, 2019, 03:50:49 pm
content: Quote from: m3vuv on September 09, 2019, 01:13:35 pmNoIt's crazy enough thinking of 10 second delays using a 555.The accuracy, the sensitivity to leakage and change of the electrolytics will kill you.You want to fine-tune for 10 seconds? You might need 10 cycles to get it straight, that is 10*(10 sec+10sec) = 3 minutes * 100 units = 5 hoursOh, a 24 hour cycle? It will take you a week to adjust each unit.Also, how would you put each unit in operation when you set it up at a random point in the 24 hour cycle?If it were just to be on for X hours out of every 24, a mechanical timer would be the easiest, cheapest.If it needs to cycle at a proscribed rate for X hours out of every 24, you had better use a microprocessor with a display so that you can set (and verify) the current time, the active times and the cycling parameters.Also, this whole project is a moving target.The next requirement will be that the unit shuts down earlier on Sunday and holidays.

author: m3vuv
author_rank: Super Contributor
timestamp: September 09, 2019, 08:32:53 pm
content: im afraid it isnt me changing the goal posts,but the water board,the 6hr time cycle isnt critical,an hour either way repeatability would be ok,this unit has to fit inside a manhole in the road and just have the chemical bag and batterys swapped weekly,the reason they want it shut off at night time is they dont want to be dosing the sewer at off peak timeswhen the flow is low,cost isnt an issue within limits but battery life is.,cheers m3vuv.

author: Renate
author_rank: Super Contributor
timestamp: September 09, 2019, 09:44:55 pm
content: You know, this is a complicated project and if you were more up-front it would be a lot easier.You never mentioned battery operation.If you run a standard solenoid valve (2-5W) and keep it open for 12 hours you will be using a ton of electricity.You either need a motorized valve or a peristaltic pump.How are you even going to get consistent flow rate with just a valve anyway?Have you picked out a valve?What is the total volume that this thing is going to dish out each week (i.e. each battery charge)?

author: m3vuv
author_rank: Super Contributor
timestamp: September 10, 2019, 12:22:43 pm
content: the solenoid valve controls an air valve that controles an actuator thats powered fron compressed gas and delivers 2ml per pulse,looking to use a 6ah sla for the power to the timer and air valve,target is have the bag of chemicals last 8 days ,then swap the sla for a charger one along with the bag,needs to fit just below the top of the manhole so no ba kit is needed to swap the bag etc.

author: m3vuv
author_rank: Super Contributor
timestamp: September 10, 2019, 12:24:19 pm
content: latest plans.

author: Renate
author_rank: Super Contributor
timestamp: September 13, 2019, 06:54:23 pm
content: Well, this is all going to be a huge undertaking no matter how you slice it.Going around and lifting manhole covers all over town once a week?The pneumatic idea is cute, but do you really need athirdexpendable to make your life difficult?(1: battery, 2: chemicals)2ml/10seconds *12hour/24hour * 1 week = 16 gallons.Have you actually tried any of this with a PC as timer, using your valve solution and measuring the current?(16 gallons of water = 60 kg)

author: m3vuv
author_rank: Super Contributor
timestamp: September 14, 2019, 03:56:28 am
content: not yet as waiting for the vale to arrive,its all up in the air untill i get some parts to take measurments,the water ppl are pushing to have us get a unit for then to trial in 4 weeks!!,talk about the heat being on.

author: themadhippy
author_rank: Super Contributor
timestamp: September 14, 2019, 12:33:44 pm
content: Quoteonly in the usa , its 72kg in the uk

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 14, 2019, 04:42:19 pm
content: Quote from: themadhippy on September 14, 2019, 12:33:44 pmI guess it's obvious given the context, right?