What I have used is those SOT23-8 one-shots at $0.22 USD a pop from 
fairchild. Set the length
of the O/S to 10mS, and program the PIC10F to go to sleep when it is 
done, to save power.

The Q output of the O/S powers the PIC10F.  The current needed for these 
devices is very, very low.
They operate at 2V just fine.

I must ask you another question... I have rejected the PIC10s because 
they have no BROWNOUT
detect, which I think is important for key tasks like this. Do they 
startup reliably?

--Bob


Rolf wrote:
> 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
>

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