[EC] Informal Meets: Think and Tinker sessions

[Ankita Pasad]

Hello Tinkerers!

Electronics Club, STAB proudly announces the commencement of Informal meets to bring out the tech enthusiast in you. The idea behind an Informal meet is simple.

• Ideate : We come up with an idea of interesting and useful/fun project
• Discuss : Share the idea on the group and discuss on how to go about the project.
• Build!!! : When completed with ideation, start building the project in one of the informal meets

Feel free to share your own ideas on the group. We will bring the ideas to reality in a couple of sittings. The components and required accessories will be provided by electronics club. 

For starters, electronics club suggests making an L293D Tester. Motivation behind this is problems faced by freshmen in the motor-driver circuit they use in XLR8 competition. Following is one of the simpler designs for the tester.

Give 5V at all the i/p pins - Check if all outputs are high using an AND gate

Give 0V at all the i/p pins - Check if all outputs are low using a NOR gate

And the led will glow only if both these conditions are satisfied.

So individual blocks are

- Clock generator (1-bit counter)

- L293D with necessary ICs at output (logic gates)

- Output signal (in the form of led/lcd display)

-(if needed) pins to check the actual voltage given by the IC.

Please share your own designs/suggestion/improvements for the L293D Tester as we have kept it open for your discussion.

Cheers!

Ankita Pasad

Manager, Electronics Club

[Jayesh Hannurkar]

A simple and rather primitive design for the L293D tester can be made by using the fundamental of a scale.
A circuit that compares the output of one L293D against another can be easily implemented and the fun thing would be that we won't even need a single IC for this... :D

A basic circuit that can achieve this is shown below

The only downfall of this circuit is that we will always need another L293D for the comparison, but I guess that it something which can be easily achieved given that the vision of this design is to use it during XLR8.

[Nimit Kumar Singh]

@Jayesh: According to your design I guess we would need 4 such LED pairs(for each output). => 8 LED & 2 L293D

I propose (4 LED & 1 L293D)

1) Have only one L293D and 4 LEDs each going from one of the output pin to ground and apply desired test case at the four inputs.(We are just replacing multimeter by an LED :P). 

2) Or you can just short the four inputs together and apply either high(All LEDs must glow) or low(All LEDs must be off).

3) Or if you want user to be more comfortable go with what Ankita had said.(The output is pretty easy to interpret)

PS : Series current limiter resistances should not be forgotten(120/150 ohm)

[Nimit Kumar Singh]

@Ankita: What are we doing next? Should we finalize one design and make it!

--

Nimit Kumar Singh
Convenor
Robotics Club
IIT Bombay
+91-9769563411

[Ankita Pasad]

Yes we finalized upon the design attached and have also made the eagle design.

Do let us know if you suggest any changes.. 

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[Nimit Kumar Singh]

Two things:

1) Final OR gate can be made by a NOR followed by a NOT(made out of yet another NOR) hence using all four NOR efficiently and no need of OR.

2) NOR gate should be open collector to drive LED(7433).

One drawback which I see:

In the extreme case this design would fail i.e If you apply high at 4 input pins and L293D produce low at all output pins then also it will glow the LED saying that IC is ok similarly in the other case too. This is the result of oversimplifying the design :P
A suggested work around : Use the previous design I proposed which will save logic gate ICs as well this bug.(Though will use 4 LEDs and a bit more intelligence on users side and won't look that cool :P). You choose :)

[Riddhish Bhalodia]

First point is good, will definitely keep in mind. Open collector does not cause problems, LED is a good device it works, we have actually tested that :)

[Ankita Pasad]

@drawback - fair point Nimit.

We can just remove the OR gate at the end and have 2 leds instead of one; this will solve the glitch.

On Thu, May 15, 2014 at 9:54 AM, Riddhish Bhalodia <ridd...@gmail.com> wrote:

First point is good, will definitely keep in mind. Open collector does not cause problems, LED is a good device it works, we have actually tested that :)

[Nimit Kumar Singh]

@Riddhish : I was just worried whether the AND and the NOR gates would be able to drive the LED. The output current rating of 7402(NOR) and 7408(AND) gates is less than even 1mA but the LED will need about 10-20mA. To supply that much current one alternative is to use open collector gates. 

open collector ICs : 

AND : 7408 (Available in WEL)

NOR : 7433 (Not available in WEL)

Please suggest if you come across some other work around.

@Ankita : The solution seems nice. :) btw why aren't we going for just 4 LEDs connected as indicators, it will even tell which in-out pair is not working rather then this whole logic there? That circuit will need 2 more LEDs and 2 less ICs :P

[riddhish bhalodia]

No 4 LED is a good idea, we are going for it.

Riddhish Bhalodia

Manager,Electronics Club

Indian Institute of Technology, Bombay