Cheap Latching LED switch -- 4 components

[MrAl]

Hello,

Here's a latching LED switch that uses two very
small momentary push button switches to turn
a high current LED on and off.

The FET and the 2N2222 transistor can be replaced with
generic devices if desired.

http://hometown.aol.com/xaxo/page7.html

Take care,
Al

 

[tonyb]

I guess it runs off of 6 volts. I need something that will work down to .8 volts.

 

[LED_ASAP]

You can get a single-device version by using a SCR (ok, three devices if you count the push buttons) and it will work from a single cell. You do lose a whopping 0.7V, though.

 

[CNC Dan]

[ QUOTE ]
LED mods As Small As Possible said:
You can get a single-device version by using a SCR (ok, three devices if you count the push buttons) and it will work from a single cell. You do lose a whopping 0.7V, though.

[/ QUOTE ]

And your switch(one of them) needs to be able to handle the full load curent.

 

[LED_ASAP]

Not really. The "on" switch only needs to handle the trigger current which is no more than 1mA. The "off" switch needs to handle a pulse of the full load but it doesn't have to be rated for the full contenueous load.

 

[CNC Dan]

[ QUOTE ]
LED mods As Small As Possible said:
Not really. The "on" switch only needs to handle the trigger current which is no more than 1mA. The "off" switch needs to handle a pulse of the full load but it doesn't have to be rated for the full contenueous load.

[/ QUOTE ]I am thinking of the SCR circiut that uses a NC momentary switch in series with the SCR and load. Pressing the switch breaks the circiut and allows the current to fall below the SCR's latching current. This configuration would have the full load going through the off switch.

Is there another arangement that has the off switch in a diferent place?

 

[Doug Owen]

[ QUOTE ]
CNC Dan said:
Is there another arangement that has the off switch in a diferent place?

[/ QUOTE ]

You can also use a switch to 'short out the SCR', which if it is low enough resistance, will lower the SCR current to under the 'hold current' (current where the combined gain of the 'phantom PNP and NPN' is greater than one), shutting it down.

It takes a switch that needs only carry full current a short time and has only to interrupt full current (tough on the 'break' contact performance), it doesn't deal with 'make' current and arcing.

Doug Owen

 

[CNC Dan]

I never thought of NO switch shorting the SCR. I wonder if that is any harder on the contacts then a NC switch opening.

 

[gwbaltzell]

How about just using a SCS (silicon controlled switch) which gives access to all 4 layers of the SCR so that you have an on gate and an off gate.

 

[Doug Owen]

[ QUOTE ]
gwbaltzell said:
How about just using a SCS (silicon controlled switch) which gives access to all 4 layers of the SCR so that you have an on gate and an off gate.

[/ QUOTE ]

Or you could always just use two transistors. FWIW, you can also shut 'em down by reverse driving the gate (push it way positive WRT the rest of the device). Still, we're left with the (relatively) high Vsat. That is other schemes will get under .1 Volt, far better than four layer devices.

Doug Owen

 

[MikeEvans]

I did similar with scr 3/$1 from allelectronics scr circuit

 

[MrAl]

Hello there,

Yes the SCR circuit should work ok if you dont mind
the extra 0.7v drop.

I've done switching circuits like that using a low
sat transistor to turn off the scr. The transistor
shorts out the scr just long enough for it to
turn off.

Take care,
Al

 

[gwbaltzell]

OK, what I had in mind the moment I saw MrAl's circuit was basically the same as this High Current MOSFET Toggle Switch with Debounced Push Button. In fact had just gotten back from ratshack with a MOSFET and decided to look at some 555 circuits for something else before I breadboarded the switch.

Yeah, its more passive parts (you could leave off the power on reset), but it only needs one switch.

Best
George

 

[tonyb]

Whats the idle current draw of the circuit in the off position? Will it work on three volts or less?

tony.