This is a multi-part message in MIME format.
--------------010901020602030003060803
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Hi all.

I am (slowly) learning this electronics thing, but before I build a 
circuit, I want to run a "debounced" power supply past people who know more.

The application is a wireless remote control for a camera system. The 
control will transmit a start bit, and 8 data bits using manchester 
encoding over a 418MHz wireless link. I have the firmware working, and a 
working prototype circuit built in a breadboard. The core component is a 
PIC10f206. The intention is to run the system from a 3V lithium coin 
cell. I intended to put a momentary switch on the +3V line as the 
trigger for the remote. When power is applied, the 10F enables the TX, 
waits 1ms for it to settle, then transmits the data at 10kbaud. it then 
powers down the TX, and goes to sleep. The entire "awake time" is less 
than 3ms.

Unfortunately the switch bounces, and I sometimes get garbled 
transmissions to the RX.

I figured I could solve the problem by putting together a "bounce-proof" 
switch. I have attached a schematic.

Using a P-Channel high-side switch, with the gate connected to the +3V 
via a parallel R and C. The momentary switch is connected between the 
gate and ground. The idea is that with the switch open, the resistor 
will bring the cap to equilibrium at +Vcc, thus putting 0Vgs on the 
gate, and switching off the p-channel mosfet. There will be no current 
drain from the battery then because of the high impedance of the mosfet.

When the switch closes, it "discharges" the cap (effectively) 
instantaneously, bringing the gate to -3V relative to the source, and 
causing the mosfet to conduct. It will conduct until the switch 
re-opens, as well as the time it takes for the cap to recharge to above 
the Vth of the mosfet. The "recharge" time will be proportional to the 
resistor, and cap.

I figure I need the mosfet to conduct for 10ms at least. Once the PIC 
goes to sleep I have measured that it consumes about 25uA, so I am happy 
to let it just sit there since I will have about 7000 hours of battery 
life at that level... ;-) All the mosfet needs to do is to conduct 
through the bounces.

My math indicates that with a 3V supply, and a -2V Vth on the mosfet, 
and that an RC time constant is for about 2/3 charge of the cap, and I 
need 1/3 charge to cut-off, that the RC constant should be about 20ms 
for the RC. Using a 1MOhm resistor (to reduce current draw when the 
switch is closed to 3uA), I need a cap of about 20pF.

The questions I have are:
1) will this system work reliably (it appears to work at 5V in my 
breadboard now)?
2) 20pF seems real small, I am thinking I should allow more current to 
flow, and use a larger cap, but this will have (small) implications on 
battery life. Perhaps I should go with 100K and 0.2uF, allowing 30uA 
current to ground if the switch is held closed (remeber, this is going 
to be jammed in a camera bag somewhere, and could potentially be held 
closed for a long time...).
3) Although I can find a number of SMT p-chan mosfets that have a max 
Vth of -2V, I can't seem to find parts that are Thru-home (can put in to 
breadboard...). At worst case I will just solder some leads right on to 
a DOT-23 package or something, but perhaps someone knows where I can 
find a -2Vth p-mosfet that is readily available (digikey hopefully) in a 
t-hole package.

Thanks in advance for any insights/criticisms.

Rolf


--------------010901020602030003060803
Content-Type: image/gif;
 name="Power.gif"
Content-Transfer-Encoding: base64
Content-Disposition: inline;
 filename="Power.gif"
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--------------010901020602030003060803
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

--------------010901020602030003060803--