ULN2803 DC Motor Control

[lrr81765]

I have a 12vdc motor that has a forward connection and a reverse connection. The motor shuts off when it hits a limit switch ( this is for a valve forward is open, reverse is close)
The motor is 55ma max.

I am trying to emulate a SPDT circuit here using as few components (and cost) as possible. Saw some really nice Max chips, but a $7 price point kills it

So, I have the attached circuit. When the input is LOW, it should go forward and when it is HIGH it should reverse. It only has these two states.

Can someone review this circuit and let me know their opinion ?
THanks

[Alberto]

Can someone review this circuit and let me know their opinion ?

Nop, your circuit will not work! To reverse a brushed dc motor you need to reverse the polarity. You can achieve it with an hbridge which needs NPN and PNP transistors.

Google for hbridge and you will find several examples.

Alberto

[SgtWookie]

Actually - is your motor a linear actuator?

Some linear actuators have a +V input, and a FWD and REV inputs. Some have a GND you also need to connect. You run the actuator in FWD or REV by grounding either the FWD or REV input.

Are your motor's connections similar to that?

If so, you COULD use the ULN2003 to switch it. However, you need to keep in mind that the ULN2003 has open-collector outputs; this means that they can sink current (apply a ground) but cannot source current (supply current).

Make certain that you connect the GND terminal of the ULN2003 to ground.
Since you are driving an inductive load, you should connect the COM terminal (opposite the GND terminal) to Vcc; as the COM terminal is connected to the internal protection diodes. If you don't, you may blow the Darlington transistor output.

I see that you are using a CMOS 4069 as an inverter. ULN2003's are really designed to be driven by TTL-level logic (0v-5v). You really should use ULN2004 or ULN2804 drivers instead, as they have 10.5k input resistors instead of 2.7k input resistors. This is much less of a strain on 4000-series CMOS components.