On Sun, 01 Feb 2004 21:50:45 +0000, Mike wrote:
I need to switch a very small 5vDC load with a 110vAC trigger. I wish to
keep my solution simple, and small. The 5vDC load is actually the button
contacts on a PC joystick. With my limited electronics experience, I
would typically use a transformer to get the 110vac down to something
usable (like 12vdc). Then, I would use a 12vdc relay. While this is
simple, it is not small. Is there a single component solution, or even an
IC that could do this?? I was looking for a small (less than 1" cubed)
110vac relay but could find nothing (my local dealer said that there isn't
really an application where 110vac is used to switch something that small,
therefore most 110vac relays will be larger to handle a larger load). Any
suggestions appreciated.
-Mike
Not a single-component solution
AFAIK. I could get it down to 2 though...
An AC opto-isolator (i.e. one with 2 leds in it) fed from the 110v line
via a capacitor to limit the LED current. You can calculate the value of
the capacitor! Look up the formula for capacitive reactance and use your
normally calculated LED dropper resistance to find the value of capacitor
needed. This has the advantage over a resistor of not needing to dissipate
a lot of heat.
Go on then - here's the formula:
Xc= 1
--------
2 PI F C
.... and here is the rearrangement to find C:
C= 1
---------
2 PI F Xc
Note: Xc is in ohms and C is in Farads
Try running the opto-isolator input at about 2-3mA.
It might be a good idea to shunt the capacitor with a high value resistor
(say 470k) to discharge it when the supply is switched off.
If you can't find an AC opto-isolator use an ordinary dc one with a diode
(1N4001 is ok) connected in reverse-parallel with the input.
You may have a problem with any opto-isolator system though. The output
from the transistor will be in the form of pulses while the input is on.
Unless you include software filtering, or the PC input is sluggish enough,
these may look like a *very* fast joystick! The ac opto-isolator will
produce a pulse on each half-cycle whereas a dc one will produce a pulse
on each cycle. You stand more chance of success using the AC version.
BE CAREFUL! YOU WILL BE WORKING WITH VOLTAGES THAT COULD INJURE YOU!
---and don't let the 110v into the joystick port! :-)
--
Mick
http://www.nascom.info for Nascom & Gemini information
Also at
http://www.mixtel.co.uk where the collection started.
Currently deserting M$ for linux... :-)