very simple time delay circuit

I appreciate all the suggestions, but I have to make this real simple as I have a "Fleet" of 20 of these streetcars to alter. I can use these small light weight 12 volt security batteries, but how long will they last, I mean how much energy do these time delay circuits consume.....thanks again

Richard,

Everything is a trade off between effort, time, and money.

What do you consider "simple"? You have a simple solution: A timer module, a battery, and a switch. What part of the solutions suggested do not meet your concept of a smaller battery, a smaller circuit, and operates as you described?

The 555C and transistor circuits will need several mA of current and drain your security battery in no time., so require a bigger battery than that. The PICAXE circuit will draw about 0.5mA@4.5V, and it might be able to be made to draw current only during the pauses. But then, it requires programming.


Ken

so how long will it have to go between charges. most solutions will comsume but a gew mA or less. just think of it the power required by a comparator IC and the negligeable loss in a ceramic condenser + the negligeable power a mosfet may use now how more efficient do you want it than that ?

the LM139/239/339 comparators use 0.8 mA so call it 1 mA to cover other losses in the very simple time delay circuit - now what can you get bettewr than that ?

Thunderchild,

Something like this? Quiescent current in the 10uA range...and then, powered only at the STOP.

Ken

yes like that not sure what quiescent current means though

Quiescent current is how much current the chip itself uses to work. 10ua is damn small. Many standard opamps draw several ma's. That's current independent of the output load.

The major current draw with that circuit would be that necessary to briefly charge the GS capacitance, Icss.
Ken

well then it looks like the original poster has obtained the answer he was seeking

Probably not. I think making a prototype to verify that it works, and then making 20 of them is not in the OP's definition of "simple".
But I could be wrong...I think I might have been once...maybe...no, I guess not!

Ken

KMoffett said:

I like Hero999's transistor monostable circuit, with high-side switching. But, since it has no values, I wonder how it will work for a 20-second period, and with less than 1.25v from Richard's nicad battery voltage.

With that design in mind, I modified my dual monostable to a single 555, moved the microswitch out of the motor circuit, and added a low Rds logic-level, P-channel, high-side MOSFET.

Click to expand...

If 6V is used as a control voltage, a 7555 will last for 60k hours when run from four AAA alkaline cells which will probably never need replacing.

The component values for my astable circuit will depend on the current consumption of the motor.

Ideally R3 should allow 1/10th of the motor current though Q3's base in order for it to saturate well.

From Wikipedia:
Multivibrator - Wikipedia, the free encyclopedia

[latex]t = ln(2)\times R2 \times C1[/latex]

But:
R2 needs to saturate Q2 so it needs to allow 1/10th of Q2's collector current.

R1 needs to be small in relation to R2.

Given these rules, I've realised that a buffer is required because the capacitor value will need to be huge; calculations are shown below.

Suppose the motor requires 500mA@1.5V.

Q3(Ib) = 50mA
R3 = (1.5-0.7)/0.05 = 16R
Q2(Ib) = 5mA
R2 = (1.5-0.7)/0.005 = 160R

[latex] C1 = \frac{t}{Ln(2) \times R2} = \frac{20}{ln(2) \times 160} = 0.18F[/latex]

The solution is to use a couple of buffers to increase the resistor values. A fairly large capacitor is still required but at least is isn't super capacitor sized.


So for a 500mA 1.5V motor:

R6 = 16R
R5 = 160R
R3 = 1k6
R4 = 16k

[latex]C1 = \frac{20}{ln(2) \times 16 \times 10^3} = 1800 \times 10^{-6}F[/latex]

Of course 1800µF isn't a common capacitor value. If R4 = 13k C1 can be 2200µF which is an E3 value. R6, R5, and R3 can be 15R, 150R and 1k5 respectively.

A 6.3V 2200µF capacitor is not a very big capacitor, 10mm by 16mm

https://www.electro-tech-online.com/custompdfs/2009/05/0900766b80800481.pdf

If high quality transistors are used e.g. ZTX1048A (which saturate well at Ib/200Ic) then the capacitor can be even smaller or less buffering can be used).

So are you going for a 12V battery now?

Single 1.5V cell circuits are often quite challenging to design but I thought I've run through it for educational purposes.

Hero999 said:

Here's a circuit that will work from a single AA cell.

how much current does the motor require?

Click to expand...

Short analysis of circuit: