Making 2 L293D's work off one 14M chip

[Gaetano]

Hello all

I am wanting to make a infrared remote controlled toy using a 14M chip and 2 L293D driver chips. The code isn't a problem, getting the motors to run at the same is. The motors are rated at 1.5 - 3v. I have provide both L293D's with thier own power supply. Is there a certain configuration that I have to follow when you hang 2 such chips off a 14M. Thank you for any help that you can provide me.

Regards
Gaetano

 

[MPep]

As long as you keep a firm control of the sets of output pins, per L293D, then no problems should occur.
Try it .

 

[westaust55]

If the motors are simple DC 2 wire motors, why are you using two L2993D chips.

It is because the current draw is high and worried about heating?

If you look at PICAXE manual 3 (currently V4.3) on page 12 there is a connection diagram for two motor on one L293D

Edit:
If you need more help it would be good if you could upload your circuit diagram and existing program code so we can see what exactly you are doing.

 

[Gaetano]

I have actually tried it but when I connect two, neither work. I tried to increase the supply to the L293D to 6v and then the single set of motors did nothing, as soon as I lowered the supply to 4.5v then these motors worked. When I tried to add the other L293D to the circuit it all went quiet and nothing happened. Is the fact that with 2 L293D's in a circuit the circuit becomes unworkable?

 

[moxhamj]

This all seems most odd. Maybe a bypass capacitor issue, or a return earth path that is resetting the picaxe. Any chance you could please post a photo of the setup?

 

[Gaetano]

I have made it on a bread board for my students to get an appreciation of the wiring configuration. I will try to take a photo tommorow and post it. As another option I am trying to link the 14M directly to the 2 L293D's through premade PCB's and using three separate 4.5v power supplies to see if this resolves the issue. Thanks for your feed back

 

[SAborn]

Just make sure you tie all 3 power supplies grounds together or you will have allsorts of wierd things take place, if not damage to the chips as well.

Are you using the enable pins on the 293d correctly, if not nothing will work.
The enable pin needs to be high or the outputs will be switched off.

I gather they are a DC motor (2 wire) and you should use output 1&2 with enable 1 together for 1 motor, and output 3&4 and enable 2 for the second motor.

If input 1 is high then input 2 will need to be low, or vise versa for reverse motor direction.

 

[boriz]

How many motors? Are you piggy-backing (paralleling) the 293’s for extra current? Are you using PWM? Are you sure it’s wired up correctly? Please post schematic+software.

 

[Rickharris]

L293's current limit at relatively low levels. This may be your problem. Many "toy" motors will draw an amp or so on start up or if heavily loaded.

 

[slimplynth]

Ricks comments would be my experience too, was attracted to the L293D because of it's ease of use/simplicity but eventually settled for a couple of relays and transistors to make an H Bridge. (The L293 produced erratic behaviour but that was obviously due to the selection of motor on my part and not looking at datasheets )

 

[Rickharris]

This might be a good point to repeat the principles of using relays as an H bridge for the new starters out there. Skip if familiar.

An H bridge is a way to provide suitable voltages to a DC motor to make it turn forward and reverse. This can be accomplished with just 2 outputs from the Picaxe. Lets choose output 2 and 4.

2 low 4 Low Motor stationary
2 High 4 High Motor Stationary
2 Low 4 High Motor turns one way
2 High 4 Low Motor turns the other way.

In general the motor can have a separate power supply (in fact usually recommended)

The L293 is one of several ICs that provides this function. H bridges can be made from transistors and FETs circuits abound on the web.

For higher current operation (and speed of build perhaps) 2 single pole double throw relays can be used as in the attached diagram.

 

[Gaetano]

Pictures of breadboard 14M & two L293D's

Hello

Thank you all for your comments to date. I have uploaded two pictures of the breadboard. The LED's that are on represent the outputs which have a cycle of 5 seconds each. I have supplied the L293D separately with a 4.5volt DC power supply. The 14M is run using 4.5 volt battery pack. Unfortunately I cannot upload a clip of the circuit running. It would show that only the motor with the yellow pinion gear will work. The motor with the green pinion gear will not work. The main thing that I notice is the crackling sound that the yellow pinion gear motor makes when it is paused. When I try to connect the white and green wire to output pins 2 & 3 on the 14M the yellow pinion motor simply refuses to work. If anyone has any further ideas I would grateful.
Gaetano

 

[SAborn]

As i asked once before but you did not answer .....is the grounds of the seperate power supplies all connected together as a common?????
I can not see where this is so in your photos.

You dont have any diodes across the outputs for back emf and with emf from 2 motors might be causing allsorts of problems. By memory i dont think the 293D has built in diodes.

If you can post the section of code that just controls the motors it might give a better idea.

It should be only a few lines and the use of no more than 6 pins to drive 2 motors.

Like....

Enable pins ..high
input 1 ...high
input 2 ...low
input 3 ...high
input 4 ...low

Above i am refferring to the 293D input pins as i dont know what is connected to where on the 14m

As for motor OFF just take the enable pin low as the power will then be switched off to that motor and you should have no crackling sounds.

 

[SAborn]

Ok on a closer inspection.

You have power supply (VS) and logic supply (VSS) tied together and from what you said you have 2 seperate power supplies.
This is wrong... VS (pin 8) needs to be wired to the motor power supply and VSS (pin 16) needs to be wired to the Picaxe power supply (where the logic is comming from)

Also you have the enable pins tied to VS and they should be tied to VSS or better still a picaxe output so you can control them to turn the motor on and off.

To me you have messed the wiring up on the driver and yes it wont work.

As i said above the grounds or negatives of both power supplys need to be connected together. (i dont see this done)