Determining the distance
Two distance-measuring devices were tested:
- HC-SR04, an Arduino-compatible ultrasound module
- Car parking sensor
Both devices have a similar operating principle. They are different in directional diagrams, range of obstacle detection, and the way they are constructed.
Table 1 – Comparison of characteristics
|Detection distance (m)||4||2,5|
|Number of sensors in one device||1||4|
While testing we found that the HC-SR04 doesn’t detect obstacles as well works worse in challenging weather conditions (low temperatures)
Information processing and control
We chose the Arduino platform as a control board. Smaller versions like Arduino mini, Arduino Nano, or Arduino miniPro are the most appropriate choice for this task. In principle, any controller with similar characteristics can be used.
Lithium-Ion (Li-Ion) or Nickel-Metal hydride (Ni-Mh) batteries can be used to power the device.
Li-Ion batteries have the following advantages compared to Ni-Mh for optimized performance in normal climate conditions:
- The charging circuit is easy to make
- Assembled charging modules are available
- High output voltage
- Versatile sizes and capacities
Ni-Mh batteries are more suitable for use at low temperatures
The output voltage is not sufficient for all the device components so it has to be amplified. We use DC-to-DC up-converters. The input voltage is 0.9-6V, the output is 5V.
The module starts working from 0.9 V so to get 5V we need just one 1.2V Ni-Mh element. The smaller the input, however, the less stable the device due to a low loading capacity, so it’s best to supply at least 2.4V (2 Ni-Mh elements) or 3.7V (Li-ion). Also, some DC-to-DC converters only work with voltages above 3V, which should also be considered.
Charging the batteries
There are many inexpensive ready-made modules with indicators of remaining charge. Some batteries have a built-in charge controller with 4.2-5V being sufficient as an input voltage.
Ni-Mh batteries are more complicated. There are no built-in solutions available on the market. A specialized external charging device can be used or an in-house charging circuit has to be developed.
One of the ways to charge a Ni-Mh element is by using two LM317 (or similar) linear stabilizers in series. The first operates in limiting current mode, and the second in limiting voltage mode.
A fully-charged Ni-Mh element has a voltage of approximately 1.45V. The charging current is set at 100-200mA. If there is no radiator then the charging current should be no higher than 100mA otherwise the circuits will overheat and go out of order.
The advantage of such circuit is that it is not necessary to control the charging state: once the needed voltage is reached the current will drop to a safe minimum.
The input voltage of this circuit should be at least 7.5V. If the stabilizers are uncooled it is not recommended to go beyond this voltage.
Depending on the choice of the warning channel (audio or tactile) the device is either a buzzer or a vibrator motor. Both can be combined to give the user a choice of switching between them.