CyBENCH :: Robotics - Electronics Resources
Home Projects Cybot ScrapBots Part Salvage Site Map Links
  PROBLEM PAGES
Problem Index
  SITE INFO
Contact Info
Disclaimer

Possible CYBOT Low Battery problem


Following a number of posts on the various discussions boards:

Problem: Cybots keeps stopping.

Quite simply, the RED LED low battery warning system doesn't seem that good.

This may be to do with the actual quality of battery used.

Even though the RED led is off, try replacing the 9v battery.

EXPLANATION

The 9v from the PP3 is taken to a voltage regulator.
This device ensures that a good 5v output is delivered to the circuits.

A standard regulator has been used in the design - probably due to cost.
This device needs to be given around 7.5v on the input to ensure a 5v output.

As the voltage drops below 7.5v, the output voltage drops.

LOW BATTERY WARNING SYSTEM

The low battery warning system is triggered at appox. 6.5v.
This is below the minimum regulator input.
This means that the regulator output will be below 5v when the RED led illuminates.

SYSTEM RESET

A reset system, often called brown-out protection, is present on the sonar processor board.
This little circuit monitors the 5v line.

The whole system is RESET when the 5v battery level drops below a set limit.
This reset is not released until the 5v line increases above the set limit.
During reset, Cybot is disabled in every way.

This system is also used to ensure that the whole system is reset at power-on.
It also ensures that the whole system is reset at the same time.
This reset signal is available on pin 6 of the vertical connector on the processor boards.

It is possible for the 5v level to rise and fall depending on its loading.
As more current is required from the battery, the battery voltage may fall.
This means as motor control circuits are switched on and off, the regulator input voltage changes.
As this changes, if below 7.5v, the 5v output may change.
This may trigger the reset circuit and lead to a strange on-off behaviour.


Summary

The reset system can trigger and disable/restart cybot without the RED led being on.


Possible Improvements


Battery

Use a 'good' battery - not cheap batteries, you get what you pay for.


Regulator change

Better regulators are available.
These are called LOW DROPOUT REGULATORS

As mentioned, the standard regulator needs an approx 7.5v input to deliver 5v output.
Low dropout regulators typically would only require an approx 5.6v input to do the same.

These devices are slightly more expensive.
But you get longer life from your battery and more reliable operation.

Some soldering skill is required to change the 5v regulator on the motor board.



The above picture shows the location of the regulator.


I will try to find a good source of these devices and update this page
Basically, you need a 5v regulator, low dropout, TO92 package, capable of 100mA.

If anyone wants to do this or would like this done, please contact me.
I can supply regulators and fit them if needed.


I will calculate and test new resistor values for the low battery circuit.
This will ensure you change the battery only when needed.