I want to design a circuit that powers a motor on under certain criteria, and off under other criteria. I built my circuit using switches and a 74ls04 (Inverter) and a 74ls08 (AND) gates. The circuit is quite simple, and works properly when I hook the output up to an LED. My circuit runs properly off of an input of 5v, 9v, or 12v.

However, I encounter problems when I hook the output to a motor rather than an LED. The motor that I am using is a Fishertechnik component which accepts 6v-9v. When I replace the wire to the LED with the wire to the motor, nothing happens.

I used a digital logic probe to try to pinpoint the problem. When the wire connecting the motor is connected to the breadboard in the output position and NOT connected to the motor it produces a digital "high". However, when that same wire is in the same output position on the breadboard and the other end IS connected to the mootor it produced a digital "low".

I hooked the motor directly up the the 9v battery and it worked. It just doesn't work when hooked up to my circuit! But my circuit works just fine when hooked up to an LED rather than the motor (for an output).

I also tried using a multimeter to try better understand what is happening. It appears that the output of the IC which is hooked up to the motor is at .75v when the motor IS connected, but at 7.6v when the motor is NOT connected (with a 9v battery hooked up to the input of the circuit).

I consider myself a beginner with electronics, so any help you may be able to provide is great! I know that it is not my circuit, but I just can't figure out why the motor is acting so weird with the circuit, but fine with a battery!!

  • 1
    \$\begingroup\$ Also, running 74LS from 9V is not guaranteed to work... \$\endgroup\$
    – user16324
    Commented Jul 13, 2015 at 16:12
  • \$\begingroup\$ If you want to use logic voltages that high then you should be using CD4k instead, but then you will need to use a separate driver. \$\endgroup\$ Commented Jul 13, 2015 at 17:08

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Trying to power a DC motor from glue logic isn't the best idea. they aren't designed to sink a lot of current. Remember a LED only pulls 20mA or so a DC motor will pull quite a bit more. I would use the glue logic to drive a gate of a Mosfet that would sink the current required to run the motor, above is a rough schematic. If you search MOSFET low side switch, it will give you more ideas.

  • \$\begingroup\$ what about freewheel diode of the load \$\endgroup\$
    – user16222
    Commented Jul 13, 2015 at 16:23
  • \$\begingroup\$ yeah, was a rushed rough sketch, i've edited it now. \$\endgroup\$
    – B-TECH
    Commented Jul 13, 2015 at 17:03
  • \$\begingroup\$ Thank you! I was thinking it must have something to do with the current. I've never heard of a MOSFET before. I researched it a bit online and it looks like it is exactly what I need. \$\endgroup\$
    – Beth
    Commented Jul 14, 2015 at 14:36
  • \$\begingroup\$ Do I need to switch my IC's to CMOS (like a CD4081 AND gate and a CD4069 Inverter gate)? - It looked like the MOSFET needs to be used with CMOS??? \$\endgroup\$
    – Beth
    Commented Jul 14, 2015 at 14:43
  • \$\begingroup\$ Aren't there many types of diodes? What type should I use for this project? How do you determine this? \$\endgroup\$
    – Beth
    Commented Jul 14, 2015 at 14:55

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