/diy/ - Do It Yourself

Here's the timer circuit and a few rules to choosing values:
>trigger_voltage = V*R_3/(R_2 + R_3)
>maximum_delay(seconds) = P*C*ln((R_2 + R_3)/R_3)
>maximum_load_current = B_T*I_c
(B_T is the beta value of the transistor, I_c is the maximum current the comparator can source, don't worry about this if you use a FET instead, which I suggest you do if you plan on running something high-power)
>waste_current = V^2/(R_2 + R_3)
>discharge_time(seconds) = R_1*C*5
>maximum_discharge_power = V/R_1

And a few guidelines:
> V/L < maximum_load_current
> maximum_discharge_power < R_1_power_limit
> 0.2*V < trigger_voltage < 0.8*V
> waste_current < 1mA
> discharge_time(seconds) < 1s
> P >> comparator_input_impedance
> time(seconds) = 3600*time(hours)

Because of the soft limit on the discharging of the capacitor as set by the resistor's power limit, the capacitor has to have a fairly small value, so a supercap probably won't do, but something that requires a resistance value higher than a few hundred kΩ will interfere with the capacitor charging by virtue of the comparator's input impedance. So if you want the maximum delay to be around 5 hours, then you'll have to either use a power resistor R_1 significantly higher than 0.25W, or settle in for a discharge time significantly longer than 1s.

>>1216892
LTspice, it's a free circuit simulator by chip manufacturer/designer Linear Technologies. All you need to know to get started is to always put a ground reference somewhere, and start the simulation with the spice directive ".tran (start time) (total time recorded)", like the ".tran 0 3k" I've written to denote start at t=0s and simulate for 3000s. It supports all common SI prefixes but not case sensitive, so write "meg" instead of "M" because "M" is reserved for "m" as in "mA". Take some shitty GIMP circuit now.