From: email@example.com (carltons)
Subject: Re: How to get CMOS counter to drive a relay?
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Sun, 13 Oct 2002 17:50:20 GMT
Date: Sun, 13 Oct 2002 17:50:20 GMT
In article <0001HW.B9CEF278023A90B81662EAD0@news.covad.net>, DaveC
> What's the simplest method to get a Fairchild CD4020/40/60 counter to drive a
> relay with one of its Qx outputs? Is simply driving a transistor to drive the
> relay OK?
> Data sheet says output is low-power TTL compatible: fan-out 2x for 74L or 1x
> for 74LS.
> The relay coil is 12v dc, 720-ohm, 16.70 mA (Omron G6B-2114P) used to switch
> a 150 mA(?), 30vac HVAC control circuit.
> Please give component p/n's and values.
Take the output with which you want to use for driving the relay (and I'm
assuming you want to close the relay contacts) and put about a 10 Kohm
resistor in series from it to a swithcing transistor's base such as a
2N2222 or 2N3904 with the emmitter grounded and the collector going to one
side of the relay coil. Connect the other side of the relay coil to the
positive supply. Note: please make sure that your power supply's negative
is also hooked to the ground. As a precaution, also place a silicon diode
across the relay coil with the non-banded side on the collector of the
transistor and the banded end on the positive supply end of the coil.
This should suffice for most applications using small to medium relays.
If you have a power contactor type of relay, then my advise is not worth
What do you get?: When the Q? goes to "high" (level of supply voltage of
the ic), the transistor will conduct, actuating the relay. If the voltage
at the collector of the transistor is low (less than the power supply
voltage) and the relay still doesn't pull in, then you need a new drive
solution or a more "sensitive" relay. Send another message to the board,
in this case and please give us the details of your problem and your
observations of the voltages, etc.