Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

help with CMOS LOGIC gates

Status
Not open for further replies.

Jules

New Member
I have two voltage comparators based on a quad LM339. One output (temp monitor) produces logic 1 as default, and the other output (battery voltage) gives logic 0 as default. I want to input both these logic levels into CMOS gates to switch a power MOSFET controlling a 12 volt fan. There is no CMOS IC available to deal with my truth table (shown below). Can anyone help please? I need to keep supply current as low as possible. The two comparators are drawing 12mA already, which will drain the battery overnight unless reduced. Are the two unused comparators using current as well? I'm quite new to logic programming. Thanks

Logic levels (1=on, 0=off)
Temp Voltage Output(fan)
1 0 1
1 1 0
0 1 0
0 0 0

I have loads of NAND, NOR and AND gates available. Can the two unused comparators in the 339 be put to good use.
 

Sebi

Active Member
Change the inputs of voltage monitor comparator, and You can use AND gate. For lower current use simply a NAND-schmitt-trigger IC e.g. CD4093 instead of comparator. (or a CMOS comparator, but 4093 can do this, and only one 14pin package.)
 

Jules

New Member
voltage comparator CMOS

Thanks Sebi.
I thought my idea was foolproof, but......
can you give more info about using a 4093 as a dual comparator, and using its outputs to switch a power MOSFET ? Any circuits would be very useful. How does your idea handle my logic gate requirements? I know that CMOS use only microamps when not switching, so better than LM339. Thanks
 

Jules

New Member
CMOS gates

Thanks for the truth table. It's a great help.
Have you any ideas about using a 4093 NAND as a voltage comparator, like Sebi suggested, to save power and provide extra logic gates?
I've never seen a 4093 comparator circuit - they all use opamps of some kind.
 

Sebi

Active Member
The first gate work as supply voltage comparator.
FET must be a logic level triggered type.
 

Attachments

  • comp_839.gif
    comp_839.gif
    3.3 KB · Views: 608

Jules

New Member
CMOS gates

Thanks for the circuit Sebi. I'll make this up on breadboard. I found that the current of 12mA was produced by the 5V6 zener diode reference voltage - this needs at least 10mA to work, so was causing problems. I hope the 4093 will be much lower power.
Thanks for the CMOS idea. This deserves to be better known.
 

Jules

New Member
CMOS as comparator

Thanks for the revised circuit. The original one you sent didn't quite work as expected - the voltage comparator part permanently stopped the temp sensor part from working. If they were disconnected, the temp sensor worked fine on it own, without the voltage comparator. This new circuit looks an improvement on the first one. Have you got a theoretical value for the current flowing through the zener ? In my original op-amp comparator, this took 12mA (90% of the supply current to the op amp!)
I'll make this up on breadboard. I'm using a BUZ11 as the MOSFET. Is that OK? Regards
 

Sebi

Active Member
Yes, sorry, i've drawed it too quickly :oops:
The schmitt-trigger threshold depend from supply voltage, so need a zener diode. ZPD types happy with 0.5..1mA, i've tryed it. Go up the Z-voltage 6.8..7.5V, this better for BUZ11 gate.
Z=6.8V, pull-up resistor (poti+R) 200k switch off about 9.8V.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top