The subject tells the goal. Here is the rest of the story. There is no micro available. It is preferred to use a dual comparator such as
I have tried and failed. Others care to give it a crack?
John
Input Voltage?
How much "stand-by" power can be drawn by the switching circuitry?
Load Current?
...Jim Thompson
On a sunny day (Wed, 13 Feb 2008 13:15:11 -0500) it happened John Bachman wrote in :
Use a 74HC74 flipflop, it has reset for correct power up too. Screw the comparator.
9 volt battery supply. Load is 12 ma. I figure a pass transistor operated by the switch and/or circuitry will be fine.
Ideally the switching circuitry is on the load side of the pass transistor, hence no quiescent draw. That may not be possible. If it must draw quiescent current then I will have to assess the feasibility of it. Two switches is not the end of the world, just not as elegant.
John
Yeah, I know. Sigh.
John
Is there any reason why you can't use a straightforward single pole toggle switch? It is cheap, ergonomic to operate, draws no quiescent current when open, gives negligible volt-drop when closed - and is self-indicating.
If they had only just been invented, they would be regarded as one of the biggest steps forward in technology of the past 30 years.
Why not? Bad design!
Yes, it can't turn itself off.
Except for wasting batteries. Auto off is the second biggest step forward ...
Go down to your local hardware store. They have lamp switches that push on, push off. All done mechanically.
Unless the item is a camera which switches itself off just in time to miss as the shot you have been waiting for !
But many customers just push on and leave on battery devices. On second thought, I am going to randomly disable the auto-off feature. I am starting a batteries on-line business.
But the camera button (trigger) should be auto-on as well.
And if you need to switch more current than the lamp switch can handle, use it to operate a power relay. BTW, if you go the 7474 route, be sure to debounce the switch.
Tam
Something like this?
|-- 100k ----------|+/ | | | |/ | | | | | ----- 100k --- 100k | GND
The comparator is self-latching, after power-on output is high (for minimum current drawn). Alternative: connect the 10n to +, then it starts with output low.
Pressing the switch less than 1 sec switches the state. Repeating the switch presses fast does not switch state (slow debounce).
Arie de Muynck
Cost is the only objection. A tactile momentary costs .16. A toggle is a buck or more. When the total product part cost is $12 then $.84+ is lot.
Two momentaries cost $.32 and will do the job, but Oh, that is ugly.
John
I'd be willing to pay the extra for a decent switch, but I suppose it depends on which sector of the market the product is aimed at.
On a sunny day (Wed, 13 Feb 2008 23:13:22 +0100) it happened "Arie" wrote in :
I must say I am impressed, a nice solution. As to the issue what is 'nest', this uses 10 components, 2 of which are capacitors.
Placing, board size, vias.... I did say 74HC74, but even that requires some debounce, as others have pointed out, so at least 2 more components.
The OP says he has no micro. But how a about a simple 8 pin PIC? Use the internal osc, internal pullup at an input pin, trigger interrupt, flip output, hang some milliseconds in interrupt routine as debounce.....
It is all there, with *1* component, say a PIC 12F629, only 67 cent in volume.
And, it also has a build in comparator that perhaps can replace some part of the rest of the circuit.....
I would definitely go that way, have been using that PIC to replace simple circuits no several times. That way you need to have only 1 part in store.
Actually, that is not that bad, because you use the other half of the HC74 package to debounce. However, to do it right you need a SPDT switch: no capacitors needed.
Tam
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