hello,

I am new to electronics and need a bit of assistance with designing a circuit for a robot I am making. I have very little experience in designing circuits and I am relying on your expertise to get me through it.

I need schematics for a circuit that will automatically make a motor go forwards and backwards. I guess you can use capacitors to make the motor reverse on a predictable cycle, right? I don't need anything fancy. I simply want a switch to turn the whole circuit on, and then the motor should automatically go forward for a second, and then go backwards for a second. (if possible, i would like to use one of those knobs that can allow me to change the delay before the motor changes directions).


thank you very much for your time.

videogame

 

I assume you have a simple DC motor.
A capacitor cannot change the direction of a motor. At best, it can filter brush noise.
To change motor direction, you must change the polarity of the DC voltage going to the motor. This is done by swapping the motor's terminals against the supply voltage. An H-bridge composed of 4 transistors, or a DPDT relay will do the job.

 

what I think he is talking abt is using a cap is say a 555 timer circuit such that after x seconds this chip changes its output from LO to HI,

This logic signal can then be passed to the H-Bridge power-circuit to change the motor's direction

yep that would work fine, have a search for H-bridge and 555 timer circuits. a bit of "glue" logic and you have your motor revering circuit

 

i believe that this will be most suitable.

yes, i am using a simple DC motor (small) and i simply need it to go forwards and backwards.

could you please show me some schematics for this H-bridge?

i would prefer building it out of simple components (capacitors, transistors, resistors) if possible

again, i am looking for something very simple...

 

The simplest H-bridge would be an IC..
google for L6203, a H-bridge by sgs thomson that can output up to 5A

 

i have found this: http://home.att.net/~wzmicro/6203drv.html

it looks pretty complex and i have no idea where to get those parts. Is there no way to use simple components like resistors, capacitors, and transistors to achieve the same end result?

sorry if i sound stupid, i am a complete electronics n00b...

 

i have found this schematic and it looks simple to build... how will this work and how can i make my motor change directions automatically?

 

This is really all you need


The enable signal enables the chip (to turn the motor on and off), and the IN1 and IN2 inputs choose the direction...

Any decent hardware supplier should be able to get this chip...

You can make it with discrete components too, but it won't be as simple as a 'complete bridge in a chip'

also, what are your current requirements?

 

You need diodes on every transistor if you want to use this, to prevent your transistors from blowing due to back EMF...
Also, its a rather strange setup, i'm not sure it will work this way...

To make it reverse by itself you could build a 555 timer circuit (lots of scematics if you google). calculate the 555 setup for a 0.5Hz frequency and a 50% duty cycle (that would make the output high for 1 sec, low for 1 sec, and so on...).
This output could then be fed to your H-bridge. if you use the L6203 bridge the 555 output would be connected directly to IN1 and to IN2 trough an inverter.
You could then use a normal switch to have a on/off function for the entire setup

 

yes, it simply needs to reverse the motor every second or so. (the motor is always running)

00:01 forward
00:02 backward
00:03 forward
00:04 backward
....continues until you open the switch

 

read previous post, my edit was too late :lol:

 

Just a thought: wouldn't it be easier to use a 555 and a DPDT Relay?

 

How noob are ya?
The simplest way is to use a latching DPDT relay and 555 timer- ever y time the 555 fires the relay changes state. The next simple way , if you can't find a latching relay is to use an ultra sensitive relay with a cap across the coil ( 100uF or so ) to ground you can get delays of a few seconds this way and when the charge on the cap drains the relay automatically opens and changes state .The constraints would be that the r/c timing constant on the 555 is equal to >2x the amount of time the cap allows the relay to stay on. The 555 fires and activates the relay . The machine goes in one direction. The charge dissipates and the machine changes direction it goes back to the original position and the 555 fires again. Trick is you don't want the 555 to fire too soon or to late. Too soon and the machine will gradually go forward too late the machine will go gradually back-wards.

The next step is discrete logic . You don't seem to be familiar with that so maybe you should look around. Theres another approach that uses limit switches. I have used these approaches on some trivial applications ( game show sets) and they work. If you have a specific goal in mind the more info would help. Theres also Potter -Brumfeild timer relays that are adjustable , but their fairly expencive.

 

That would probably be the simplest way...
A relay would also be better suited for the high currents caused by instand reversing (without braking first)

 

Very helpful and unprofessional. He has already stated he is new to teh world of electronics