Rockbox mail archiveSubject: It works! (Infrared) Remote without external power, on remote pin
It works! (Infrared) Remote without external power, on remote pin
Date: Sun, 13 Apr 2003 11:14:44 +0200 (MEST)
Diava (Dirk-Andreas) did some coding recently to get an RC5-signal from an
IR receiver decoded using the remote input pin, great job! However, I did not
like the need for an external supply to power it. I noticed that the remote
input pin has quite a strong pullup (1k towards 3.3 Volt, means you waste 3.3
mA with a regular headphone plug shorting it) and did some experiments to
power a receiver from that.
There are 3Volt receivers from Vishay, type TSOP1836xx3V, they need about
0.75 mA current. Here's the one I used:
Connecting this makes the line drop to 2.55 volt just by the supply current.
This it out of spec for the receiver, but the samples I got work down to 2.4
Volt. The problem is that a diode is needed to decouple the receiver supply
for the case that we want to drive the line low. The voltage drop of a diode
is 0.3V for a schottky, but we can't afford that. So I use a MOSFET
transistor to short the diode for the normal case where we don't pull low. This gives
the full voltage back. The diode can probably be a regular like 1N4148 again
(0.7V drop), but with a schottky the buffer capacitor charges a bit faster.
My circuit is attached, or here:
The transistor Q1 is needed to invert the signal, Q3 for pulling low only
_after_ we're not shorting the diode any more. Can't use the receiver signal
This kind of circuit can be useful as well for a little PIC microcontroller
used as a button remote, or doing the RC5 decoding outside of the box (no
change on Rockbox then). If a microcontroller is used, two of its pins can be
used to first open the diode short and then pulling the line low. Q1 and Q3 are
not necessary then.
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