Automated greenhouse project (noob) LVD circuit

[Colin Rennie]

Hello I am both new to electronics and new to this forum so if I make any silly mistakes please forgive them, Im trying my best. im still getting to grips with schematics and calculations but having a great time learning.

I am working on a greenhouse automation project based on a few other peoples ideas, a few of my own and lots of help. Im starting out simple and hopefully working up to more sophisticated ideas when I get more experiece, I have one greehouse I am using as a test site and will be building a larger one in the next couple of years, that will use arduino to control lots of parameters.

The first project im starting out with is a underground heatsink which i first saw on Dick Strawbridge's "its not easy being green" TV series. I have a 30w solar pannel and a set of 5, 12v 7AH reconditioned SLA batteries which are holding charge but unsure of their capacity, a solar charge controller 8amp with over charge cuttout, and a 12v .42a fan. The fan will blow hotter air from the apex of the greenhouse, down a plastic drain pipe into an insulated underground heatsink filled with metal, water bottles rocks etc which will store the heat during the day and release it overnight. I plan to run the fan most of the time so that it helps to regulate the temperature in the greenhouse over 24 hours. However I dont want to run down my SLA batteries below say 11.8 volts or so.

I found this LVD circuit here http://www.reuk.co.uk/Simple-Low-Voltag ... -NE555.htm using a 555 timer as a comparator with hysterisis, I didnt have a 5 v relay so I used a mosfet instead, however the curcuit logic is opposite to my needs, opening the mosfet above the threshhold and closing it out bellow the trigger, so I added a 2n222 as a not gate into the curcuit to invert the logic. at least I think thats what is on the schematic, I breadboarded it and it worked.

I would like to have a go at printing a circuit board so have started to use express SCH but Im not sure if I have the schematic or the values right here. Id really appriciate it if someone could have a look at this circuit and tell me if its in anyway right or if I have made very basic errors, (which I am sure I have)

looking forward to posting more developments when I get this project started.

Many thanks in advance

Colin

[David]

If you are using a standard 8 pin 555, pin 1 should be connected directly to ground and not via a capacitor. Pin 5 is usually decoupled to ground via a cap. Pin 4 is connected to the same point as pin 8. You should also connect a reverse-biased diode (cathode to +12V) across the fan terminals to absorb any inductive spikes. Use a diode with a current rating no less than the current drawn by the fan.

[Colin Rennie]

If you are using a standard 8 pin 555, pin 1 should be connected directly to ground and not via a capacitor. Pin 5 is usually decoupled to ground via a cap. Pin 4 is connected to the same point as pin 8. You should also connect a reverse-biased diode (cathode to +12V) across the fan terminals to absorb any inductive spikes. Use a diode with a current rating no less than the current drawn by the fan.

Thanks so much for the reply David. my mistake on the pin 1, its pin 5 that i have connected through a capacitor to ground, I messed up the schematic, I havent tried connecting pin 4 to pin 8 although ive seen that in some online tutorials, ill try it. very good tip to connect a reverse based diode over the fan terminals, I dindt think of this but thinking about it now it makes perfect sense, similar to the reverse diode over a relay, I think though Ive been lucky so far as I suspect there is such a diode already in the curcitry of the cpu fan im using, however no harm in adding one.

really apriciate your time looking at my schematic, so thanks a million.

Kind regards

Colin

[David]

Delighted to be able to help. If you would like to go into things further, there are some excellent books mentioned on this site. My personal favourite is "The Art of Electronics" by Horowitz and Hill - a bit pricey, but well worth it. You may be able to find some on EBay.