L293D used to reverse motor direction 7.8 vdc

[Doddy]

Apologies - I started to reply to this earlier before work but somehow lost my post. This is an abridged version:


I'd be concentrating in the first instance on establishing the logic supply (the +5V) to the L293 - without this you're going no-where. Add decoupling caps if you've not done so already (ref. the LM7805 datasheet for detail). If you can, measure the current drawn by the L293 - the standard 7805 can source 1A, and will current limit somewhere above that (by dropping the V-out). The L293 datasheet suggests the logic supply should be 60mA, max - above this and you have a problem - either wiring, or a knackered L293. Any metal tab on the 7805 is internally connected to common (ground/0V) - so take care that this doesn't short to something that it shouldn't.


I wouldn't even worry about the motor wiring etc until you stabilise the 5V logic supply.


If you find you keep blowing the L293s, I would consider removing the motor and channel-by-channel testing the circuit/switches with a DMM to ground.


The circuit is trivial (I mean that in the sense that there's little to go wrong) - it should be straightforward to diagnose with the above. If you report back we can take it further.

[rjsutton]

Hi Doddy
thanks for above
decoupling caps never heard of but googled and think i know where to put them. I dont have any so onwards
the output of LM708 is between 3.8 and 5v ( just changes sometimes!!!!! )
regardless the button for cw produces an output around the battery input so i guess you could say that circuit is working
BUT i can get any power out of anything when i press CCW button even tried the switch on the other side of the l293 any nothing there either
Hsvr tried 3 different L293 ans 2 lm708 and the same happens
when i do have a static current of 5v to pin 16 the same results (cw works fine but ccw nothing
Am i wiring the l293 wrong?
looked at its data sheet and all seems correct in my brain ( not sure what inverting means
all i want is to have 2 buttons
1 to make 6v motor cw push another and ccw.
Is there a better module ? i only have 23mm max width of any piece so a whole aduno breadboard thing wont fit
just tried new Lm708 and when pin 16 is NOT connected i get a static 5v. as soon as wired to 16 drops to 3.8 ish seeems strange i have a 1.2v on pin 14 and or 11 like its bleeding power to these pins ( motor connections )

[Doddy]

Hi Doddy
thanks for above
decoupling caps never heard of but googled and think i know where to put them. I dont have any so onwards
the output of LM708 is between 3.8 and 5v ( just changes sometimes!!!!! )
regardless the button for cw produces an output around the battery input so i guess you could say that circuit is working
BUT i can get any power out of anything when i press CCW button even tried the switch on the other side of the l293 any nothing there either
Hsvr tried 3 different L293 ans 2 lm708 and the same happens
when i do have a static current of 5v to pin 16 the same results (cw works fine but ccw nothing
Am i wiring the l293 wrong?
looked at its data sheet and all seems correct in my brain ( not sure what inverting means
all i want is to have 2 buttons
1 to make 6v motor cw push another and ccw.
Is there a better module ? i only have 23mm max width of any piece so a whole aduno breadboard thing wont fit
just tried new Lm708 and when pin 16 is NOT connected i get a static 5v. as soon as wired to 16 drops to 3.8 ish seeems strange i have a 1.2v on pin 14 and or 11 like its bleeding power to these pins ( motor connections )

The caps should go as close as possible to the input/output pins on the 7805 - you might find that it's possible to mount them physically to to the 7805 pins.

The behaviour of the caps is the stabilise the operation of the 7805 - which can go into oscillation in the absence of these - that is bad (I'm not suggesting that's your problem - although I could imagine you reading a different voltage on a DMM than 5V if the 7805 goes into oscillation). If I've time tonight I might try testing a 7805 with/without capacitors to observe the behaviour.

The 7805=5V dropping to 3.8V on connecting to Pin16/L293 is very significant, and I'm thinking that's the thing we need to fix.

Can you take a photo of the electronic assembly - I just want to eyeball what you've done wrt the schematic, take out any uncertainty on my side. Also, the underside of the board (I'm assuming some form of breadboard/vero board). In particular there I want to make sure that you've isolated pins on the same row on the L293 (1-16, 2-15 etc).

Re "Inverting" - the only reference on the datasheet is the non-inverting inputs on the L293 device. This simply means that if the input voltage is "high" (near 5V) then the output voltage will be "high" (near VCC2). If the input voltage is "low" (near 0V) then the output voltage will be low (near 0V),

The L293 is a suitable device for what you're trying to do.