Circuit designz ?

[cdicky]

Hi chaps / chapeses,

I'm struggling to come up with a SIMPLE circuit design for a project.
Anyone bored willing to help?

But it MUST be simple and be able to fit in VERY SMALL. Say area of 5p coin.
Dont mind smd's but cant do smd boards

All I can come up with is op-amp circuits with 741's and other bulky stuff.
Trouble is my days of college are long behind me working out basic stuff like this are a distant memory.

What I need is basically a just a switch. 12vDC in, 12v DC out.
But needs a photocell AND a hall switch.

If dark or dim (AND) NO magnet, switch ON. Bright light switch OFF.
Magnet near OFF, no magnet near (and dark) ON.
(magnet is fixed so cant reverse poles)

Should be easy but for life of me cant get the thing right.
Couple of simple bc109's and a Zener you'd think

Really appreciate any help - thanks

[budm]

Re: Circuit designz ?

So the two conditions has to exist at the same time for the switch to be on Dark + NO MAGNET (AND GATE), Or you want either Dark OR no MAGNET for the switch to be on (OR GATE). Once you select the Logic of what you want to do, then you can come up with the circuit.

[stj]

Re: Circuit designz ?

a photodiode with a reed switch in parallel feeding a cmos inverting buffer to a fet.
you can get logic in single cell 6pin smd package.

[cdicky]

Re: Circuit designz ?

Hi Guys, thanks for replies. Sorry never saw posts earlier.
As for the 'logic' thats correct. I was going to add a truth table / matrix but you got it.

No magnet / no light = ON
magnet or bright light = off.
Have searched around for simple circuits also, and can find plenty for either but not both. Would rather not use logic ic's as voltage / current needs to be dropped. More components.
Supply is 12v cant change that as its a motor circuit. Doesnt need to any finesse, just crude and brutal. ON / OFF like a switch.

Looking round there are plenty of these on ebay. Nice and cheap too ebay.co.uk/itm/A3144-A3144E-44E-938-HALL-EFFECT-SENSOR-SWITCH-MAGNETIC-DECTECTOR-UK-BRAND-NEW-/190823658126

I also found this nice simple circuit, crucially with a 12v / high current output.
I believe this could be made fairly small, but still needs to include either the hall (preferred) or a reed switch
http://amplifierlab.com/simple-photo...uit/2010/05/24

[budm]

Re: Circuit designz ?

The circuit in that link with the 555:
"The voltage across LDR is kept at 1/3 of the supply voltage by adjusting the potentiometer R2 (100K). The output of IC is high at this stage. This makes transistor Q1 turn on and Q2 will be in off state since its base is connected to the collector of Q1. As a result of this, relay will be put into inactive state. The voltage across LDR increases when light falls on it. When this voltage crosses 2/3 of the supply voltage, the output of IC goes high. By the effect of this, relay will get activated. That is relay works with respect to the intensity of light falling on LDR."
I wonder if the author actually built the circuit, I never seen LDR resistance goes up when light falls on it, same for the photo-transistor. LDR resistance GOES DOWN when light hits the cell.
When light falls on that LDR, the voltage across the LDR will DECREASE. You have to be careful on what you see on the web, best is to look at the circuit and see if it will work as advertised.
Here is a simple explanation how LDR works:
http://www.technologystudent.com/elec1/ldr1.htm



http://www.resistorguide.com/photoresistor/

Photo-transistor:

https://cdn.badcaps-static.com/pdfs/...a5e47945e0.pdf

[cdicky]

Re: Circuit designz ?

Hi yes, well spotted. I did notice that. But I just presumed they meant photo diode not ldr.

Haven't found any other circuits though, only logic based ones.

[stj]

Re: Circuit designz ?

o.k. take a fotodiode in parallel with a reedswitch,
invert the output through a transistor, then feed it into a fet.

i keep mentioning fets because you never said what the lamp current is!

[cdicky]

Re: Circuit designz ?

Current is small. 500mA or less.
Its an E.L inverter to power an EL light.
Basically a safety light for a mates cab. Not commercial. But will need one circuit for each door.

This is why prefer not to use reeds as quite big and GLASS.
Where the circuit is going its venerable to knocks, so circuit will be fitted underneath a thin metal cover.
While its ok to drill a small hole for the light switch, you can picture a glass reed poking out is risky to getting lumped with a boot or handbag or something.
I would rather a slightly bigger hole with a hall or similar flush mounted.

[budm]

Re: Circuit designz ?

Well since we do not have a good details without knowing the physical limitation, the electrical requirement, the budget, then it will be hard to come up without knowing the full details.
Not using reed switch then you will have to use some other magnetic sensor instead, just do the search on magnetic sensor and see what will fit your requirement.
You will also need 12V 500mA power supply which will be controlled by this sensing circuit and a away to turn on this power supply with relay, etc.