We now build this arrow circuit that may look complicated but it is actually not that complicated. It is made up of two major sections so let’s look at them individually.
The 555 timer is configured in the astable mode as we need to have two states – ON and OFF. R1, R2 and C1 form the RC network and just like before, the capacitor voltage is fed to pins 2 and 6 of the timer IC to control its output state. The capacitor charges through R1 + R2 and discharges through R2 only. By adjusting the values of these components, we can adjust the state times.
In order to create the output arrow effect, we add an RC circuit to the output pin. When the output is HIGH, C2 charges through R3. When the output is LOW, C2 can quickly discharge through D1, which bypasses R3. Without the diode, C2 would take much longer to discharge and this would not give us the effect we are looking for. The capacitor voltage is fed to the output stage that is made up of 5 transistors.
The transistors are responsible for controlling the LEDs and we used a similar transistor configuration in the soil moisture circuit for BBox 1. As capacitor C2 charges, the voltage across it will rise from 0 to the output voltage. Depending on this voltage, the appropriate transistors will switch ON, which will cause the corresponding LEDs to switch ON.
Remember that for NPN transistors to switch ON, the voltage at the base terminal has to be about 0.6-0.7V higher than the voltage at the emitter pin. Since we have placed D2 between the emitter of Q5 and ground, it increases the base voltage needed to switch Q5 ON by the diode forward voltage drop. The forward voltage drop of a diode depends on the current being drawn and it is generally around 0.7V. This means that Q5 will switch ON when the voltage at its base is about 1.4V. The emitter of Q4 is connected to the base of Q5, and this means that Q4 can only switch ON after Q5 has switched ON. The base of Q4 has to be around 1.4V + 0.7V or 2.1V for it to switch ON. Likewise, this continues for the other transistors to give us a sequential effect.
This is what the assembled PCB looks like. Notice that the final transistor stage is not as bright as the others. This is because the circuit is designed to operate at 12V, which allows it to be used in vehicles. To use it from a 9V power source, you would need to bypass diode D2 which will reduce the threshold voltages required to switch ON the transistors. Of course, you can even add more stages and change the operating voltage or charging rate to create the desired effect. Let’s move on to the next project.