Re: Substituting a 74HCT74 for a 4013

From: John Fields (jfields_at_austininstruments.com)
Date: 03/27/05 

Date: Sun, 27 Mar 2005 08:58:24 -0600

On Sat, 26 Mar 2005 23:27:55 -0600, Chris W <1qazse4@cox.net> wrote:

>John Fields wrote:
>
>>On Sat, 26 Mar 2005 18:29:17 -0600, Chris W <1qazse4@cox.net> wrote:
>>
>>
>>
>>>Can someone tell me how I could use a 74HCT74 in place of the 4013 in
>>>this circuit...
>>>http://www.uoguelph.ca/~antoon/gadgets/rc-sw.htm
>>>
>>>I tried adding a pull up resistor to both sets and the second reset.
>>>Then I ran the first Q output into one of the inverters on a 7404, with
>>>the capacitor to ground and then the output of the inverter into the
>>>first reset but it still doesn't seem to be working. Any ideas? I
>>>really don't know what I am doing here. Oh and I just need the high or
>>>low output, I don't need the TMOS FET.
>>>
>>>
>>
>>--- ___ _____
>>The HC74 needs a logic low asserted on the SET and RESET inputs in
>>order to make them work, while the 4013 needs a logic high to do the
>>same thing. Therefore, if you want to substitute an HC74 for a 4013,
>>you'll need to invert the signals driving SET and RESET on the 4013.
>>
>>
>>
>I guess I wasn't clear, but that is what I am trying to do. After more
>checking, I think the inverter I have isn't working right. With no
>connections the inputs all read 1.6v and it takes a resistor no larger
>than 100 ohms to take pull it down below .1v. Having a resistor in
>there is obviously going to interferer with my capacitor charging.
>Shouldn't the capacitor pull an input down to 0 volts or is this thing
>defective? The markings on the chip say DM7404N. 

---
I missed the part about the 7404 before, but that's what's keeping it
from working, since you can't pull its input down with R1 and the pot
in there.  Sinse the sense of SET and RESET is backwards for the HC74
than it it for the 4013, if you connect the unused SET and RESET lines
to Vcc instead of ground and drive the RC from Q- instead of from Q,
as shown below, it ought to work.
VCC>-----+------+-----------------+
         |      |                 |
     +-------------------+        |
     |   |      |        |        |
     |   |  +-------+    |    +-------+
     |   +--|D  S- Q|    +----|D  S- Q|---->OUT
     |      |      _|         |      _|
IN>--+------|>  R- Q|----+----|>  R- Q|
            +-------+    |    +-------+
                |      [47K]      |
                |        |       VCC
                +---->[100K]   
                |        |     
                +--------+     
                         |
                       [22nF]
                         |
GND>---------------------+
-- 
John Fields

Relevant Pages

  • Re: Substituting a 74HCT74 for a 4013
    ... >> The HC74 needs a logic low asserted on the SET and RESET inputs in ... >> you'll need to invert the signals driving SET and RESET on the 4013. ... You don't really need an inverter. ... on the first flipflop and tie the unused sets and reset to 5V instead of ...
    (sci.electronics.basics)
  • Re: Substituting a 74HCT74 for a 4013
    ... >I tried adding a pull up resistor to both sets and the second reset. ... >Then I ran the first Q output into one of the inverters on a 7404, ... The HC74 needs a logic low asserted on the SET and RESET inputs in ... while the 4013 needs a logic high to do the ...
    (sci.electronics.basics)