hi
i love combining electronics and magic, my latest project to cut a long story short,
requires a way of switching an electromagnet off and on while moving it across a surface at a distance of a couple of inches,
so i'm looking for a sensor that senses the edge of a white sheet of paper against a black background.
the trigger surface and background can be any colour,white a4 paper against a black background would be convenient
i cant have a light source on the surface, so I cant use a LDR.
thank you
doug

 

looks like infrared is the way to go,
thanks anyway.
doug

 

I would think about using something like this IR reflective sensor in the scheme of things. That assumes it can be worked into your props.

Ron

 

hi
ive cobbled togrther an output stage, for a conventional emitter/dectector circuit,
the transistor is a tip 31,gain of 25, base resistor ive calculated is 200 ohms.?
the ic is a 358 op amp, i would have thought a comparator would have been better.
im pretty rubbish at this so i would appreciate it if someone could cast an eye over it and advise.
voltage is 9 volt ,the big round thing is my e/magnet, which is taking 1 amp, see below..
i wanted a electromagnet that was flatter than normal as i need to conceal it and the circuit in the palm of my hand,after a long time of searching without success, i made my own.
unfortunately although it does the job just, its taking 1 amp and gets very warm,ive put hundreds of turns on.
although in operation it only needs to be on for a minute or so.
any suggestions on whether its possible to reduce the current without losing too much force, appreciated.
apologies for no schematic.
thank you
tibbles

 

thanks reload ron ,that looks as if it would save a lot of work.

 

Using a basic 9 volt battery with a 1 Amp load isn't too promising. The magnet current draw will be a function of the wire gauge used and the number of turns. I figure you know that. So, it comes down to making your magnet strong enough (what alloy did you use?) and keeping the current down to only what is required for whatever it is you are doing. Can you post the actual circuit rather than the board?

Ron

 

hi again
the magnet at the moment has .3 mm wire, (ive also written on the wire 28 swg/8 ?) it measures 10 mm depth x 24mm dia overall.
the magnet was just to check the feasability of such a low profile,
and is just mild steel at the moment.

i'm working on a schematic....
regards
tibbles

 

Otay!

Ron

 

 

R1 i think should be 200 r for 9 volts.

 

R1 would be Vsupply - Vf / .020 so 9 - 1.8 = 7.2 / .020 = 360 so I would use a 360 or 390 Ohm common resistor, likely the 390 Ohm. That will keep the emitter If below the 20 mA level at 1.8 Volts Vf. For R3 you can try 10K or just use a 50K pot for some room. VR1 is fine and for IC2 make sure you use a comparator. I would place L1 on the collector of Q1 and the Q1 emitter to GND. For Q1 I would use a 2N2222 which is good to about 800 mA so if the load is around 500MA will do just fine.

Ron

 

thanks Ron, i'll get back when ive put it together.
i usually spread the componants, use thick traces, use dil sockets and broadly follow the schematic, so its easy to change componants etc.
regards
doug

 

just had a brainwave, a rare event for me, i might have solved the electromagnet problem of having a low profile while still having enough length to get a good pull.
as the pic shows, a bit rough at the moment, ive bent the head of the bolt at 90 degrees, it has far more power than my earlier one, still taking over an amp though,
but it means there is enough magnetism to reduce the voltage and hence current,
and still have enough pull at say 3v
regards
doug

 

While DC electromagnets are not my forte I can share this much. The magnetism field strength is based on the number of turns and the current. There are other variables but those two are important. Increasing the turns will reduce the current as the wire length gets larger as the overall resistance increases. Since R is increasing and I is decreasing the field won't increase but the overall current for the same field strength should decrease. The smaller the wire gauge the easier to make more and more turns.

Ron