Toggle Circuit with LMC555 on less than 5V

Thread Starter

TomTommer

Joined Mar 16, 2008
4
Hello all,

I'm trying to build the following pushbutton flip-flop circuit:
http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm#555-T.gif
(or like described on http://www.cpemma.co.uk/flipflop.html )

I have a source of between 3.5 en 4.5v (3 batteries),
so I thought I could use the low-power CMOS LMC555CN.
Problem is that when I press the button once, the circuit switches on, but stays on. It doesn't switch off when I press again. :confused:

Can anyone tell me how to get this circuit working with a LMC555?
Thanks in advance (a lot!).
 

Audioguru

Joined Dec 20, 2007
11,248
An LMC555 with a supply voltage as low as 3.5V has a very low amount of output high current, maybe only 1mA. A higher load would cause its output voltage to be too low to flip the circuit to turn off. So its output must drive a darlington transistor through a high value resistor.
 

Audioguru

Joined Dec 20, 2007
11,248
Change the xstr to an NTD3055 FET (ON Semi), which has a gate threshold of 2V max.
The threshold voltage of a Mosfet is when it barely turns on. Its current is only 250uA.
The threshold voltage of the NTD3055 Mosfet is the same as nearly every other ordinary Mosfet, minimum 2.0V and maximum 4.0V.
It won't work.
 
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