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My first RC car, any advices ?

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André Ferrato

Newbie level 3
Apr 26, 2015
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Hello guys, this is my first post in this forum. I recently started making a rf rc car, as you may see it is a very simple project that i designed the pcbs in eagle and took some circuit from the web. I would like to ask some questions about my concerns at the moment.

1) I intend to use a 7.5v battery pack to run the receiver board, but i'm afraid of the drop that will occur inside the L293D chip, will it be a large drop using this motors ?
2) I am using a 10uF electrolytic aluminum capacitor at the input of the LM7805 and a 100nF ceramic capacitor on the output, i think it's a nice setup, is it ?
3) Also i am using a few jumper wires in the board, would it be good to connect 100nF ceramic capacitors near to those pins ? I am aware they will be acting like little antennas, but i dont know if the effect will something i need to consider in this project.
4) I faced some difficulties while routing the receiver board and i think this design i reached is a very poor design, if anyone could give me some tips and help it would be nice.
5) Is there anything i should add to these designs ? I am new to electronics.

PS: I know some pads are larger, some vias are larger, but this is just for making the manufacturing of the board easier. I have done it before and helped a lot. Also i am tied up to these devices, i can't change the motor drive or the encoders/decoders/rf modules.

Transmitter Diagram
Transmitter Board #1
Transmitter Board #2
View attachment J38rJyM.png

Receiver Diagram
Receiver Board #1
Receiver Board #2
View attachment Q6mzMGm.png

Motor data chart

I attached the eagle files if anyone wanna take a closer look.


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The HT encoder and decoder are VERY simple circuits for a child's cheap RC car. It operates as Bang-bang. Bang-bang is steering that is straight, full left or full right. Speed is no speed, normal speed or turbo speed. I don't remember if it has reverse.
Normal RC cars use proportional steering with servos and full motor speed control.

Why do you have two motors? Will it turn by powering a motor on only one side?

This rc car has no speed control on it, it has just the L293D H-Bridge to reverse the motors when i press the button at the transmitter, to turn the car it would be basically what you thought, by accelerating in one direction a motor and the other in the other direction the car would them turn to whatever side i chose. The idea was to make it very simple, because i am a newbie at this, as you may see, and all i want is some tips, about what i'm doing or what i have already done.

The L293D which came out in the mid 80's is a poor choice for low voltage as the bipolar outputs drop too much voltage.

Connect L293 to Vbat instead.
Vbat drops from 7.5 to 5.5 at 0% THe L293D has 3
Ω ESR so voltage applied to motor will drop under load within range but be low when battery is low.

Meanwhile for anything else that needs 5V you need a different regulator with only 0.5V drop like the TA4805BF or better.

Today MOSFET's make a better solution for half or full bridges with lower driver impedance. but you must match battery voltage to motor or expect shorter life with 7.5V on 6V motor according to temperature of motor and contacts.

examine Vol, Iol and Voh, Ioh specs in datasheet
they are worst case 1.8V drop with 0.6A or in other words an Rce(sat) = 3Ω worst case

If the motor has a no load rating of 100mA and with gearbox 150mA then you can expect 8x the no load motor 100mA current for the max surge current. ( rule of thumb) unless limited elsewhere by high Rce(sat) Thus we can estimate (unless surge current or Motor Rs is given) the Rs of the motor coils to be 6V/(8*100mA) =7.5Ω

Compared this with the L293D 3Ω and you have a total resistance of 10.5Ω but the motor will only see 7.5/(7.5+3) = 71% of the battery supply voltage when fresh. Battery ESR rises when State of charge gets near depletion and may start < 100 mΩ and rise to 1~10Ω when getting low in charge <10% (estimated, depends on your size.)

It will work and the heat lost in the chip will need to be transferred to a heat sink or large copper area on board. and may not use optimal power from battery but will work.

Mind you 7.4V to motor under load with MOSFET bridges or direct may overheat but have more speed.

Measure the motor coil resistance from the outside leads.

Where did you the 8x value from? Sorry i am new. Another thing, you're telling me to connect every pin to the vbat? Or just the onde related to the motors, because this i have already done. I thought of instead using a lm7805 that needs a constant 2v drop at 7v, another LDO, but i dont know which one. Could you give me some on this ? Normally it would need to calculate the load through the LDO, and that i dont know how to do it, or do and just dont know... sometimes i get lost in the datasheets. Also is really 100nF capacitors enough to bypass the noise ? I dont know they look so small, what calculations i could do to make sure they arent

Motors are like inductors with a series resistance Z=Rs+ back EMF raises the impedance from motion at commutation rate * inductance

A good design will have high torque limited by V+/Rs that also causes surge currents of load at peak up to 8x typically unless controlled by soft start.
This is a good Rule of THumb. Some may be lower 5x.

Another 5V reg is 78R05 with 0.5V drop is far better then 7805 (old)

Remember Zout of Op Amp rises from Ro/feedback at 0~1Hz to high Zout at GBW product due to lack of feedback.
Same with LDO's So as impedance rises from negative feedback , the opposite is true with an output cap, which is used to suppress HF noise, but not DC ripple from ratio of Motor Rs to ESR of LDO ( Check load regulation specs which are inverse of ESR)

Always understand the load ESR ( coil resistance), the power sources ESR ( inverse of load regulation error) and Cap ESR for high current ripple voltage.
Same with all semis and Diodes which when saturated have ESR approx in a 2:1 range.

If it states Vout drop at rated current, this is ESR. Or it gives worst case Vce(sat) rise at rated current, this Rce or ESR.
If it gives Stall current... this is motor Rcoil or ESR
If it gives max ripple current in big cap, this is limited by heat , max temp, thermal resistance and ESR of cap
If battery drops ?V/ I load.. this is ESR, which rises rapidly near EOL or low SoC

(forgive my liberal use of linear effective series resistance or ESR, but if you learn this ,you understand these things and much more :) even if ESR changes with aging, temperature, operating current ... the variations give you clues.

Thus never run motor from a poor LDO


The L293D which came out in the mid 80's is a poor choice for low voltage as the bipolar outputs drop too much voltage.

Yes, that's true. L293 is really an oldie and there are many chips that can replace it with great
advantages in performance.
I made a project like this one (as an after-five development, with iPhone remote-control). I have used LV8405 in the first
version, but in the latest one, I switched to DRV8448 which allows to control current.


Very prosaic point, The motor brushes will produce interference. Connect each of the motors terminals via a 1NF cap to the motor case and feed them via a ferrite bead as close to the motor as possible. Earth the cases.

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