1381-based Solar Engines
This solar engine is configured around the popular volage trigger IC-1381.This by far the most efficient solar engine. The reason being while monitoring the storage capacitor, it draws currents of order of only microamperes (better than zener SE & FLED SE which require current in milliamperes).Also, it is least tolerant to motor type, transistors & solar panels used.
About 1381: It is a CMOS voltage detector IC used to monitor power supply voltage supplied to microcomputers and is normally used to reset CPUs when the power supply drops too low for reliable operation or for preventing runaway operation when the supply voltage fluctuates. So, it detects and switches when the monitored voltage crosses the rated upper and lower threshold voltages. Say for example the output (VOUT) remains in the logic HIGH state as long as monitored voltage VIN is greater than the specified threshold voltage Vth. When VIN falls below Vth, the output is driven to a logic LOW. VOUT remains LOW, until VIN rises above Vth by an amount Vhyst, whereupon it resets to logic HIGH. This hysteresis (Vhyst) keeps input noise around the switching threshold from resulting in multiple output cycles as the transition occurs. Because of their extremely low current requirement i.e. in microamperes it is the most efficient solar engines. It’s just a three pin element with wide selection of detection thresholds from 2.0-4.9 volts. The one which we are using is 1381-E with a detection threshold of
Refer the data sheet to know more: http://www.solarbotics.com/assets/datasheets/mn1380.pdf
1381 based SE:
Working: The voltage across the capacitor rises slowly as it is charging from the output of a solar cell. This voltage also appears at the input pin (no.2) of 1381-E. The output (no.1) of 1381-E stays low until the capacitor voltage does not reach Vtrip= Vth (2.2V) + Vhyst (of 50mV-300mV). So the base of NPN transistor is at 0V, which keeps the motor in off state. Now, the capacitor voltage reaches the Vtrip & the output of 1381 switches to 0.8 Vdd (the solar supply voltage). This turns on the NPN and it starts to draw current from the capacitor which flows through the motor and causes it to rotate. The capacitor voltage starts to drop as the motor is running. When NPN turns on, Vce starts to drop and reach a potential of approx 0V at saturation. This makes the base of PNP – 0V via Resistor and PNP turns on. Ideally, as the capacitor voltage begins to drop and it reaches the Vth of 1381, its output would be 0v to stop NPN from conducting and thus stop the motor. But since when NPN turned on it also turned on the PNP which started giving the required base current to keep the NPN on and motor running . So, at this point the 1381 is out of the circuit. Now PNP gives the required base current to keep NPN on. This results in even more NPN current due to amplification by both transistors, but it also starts to drop the voltage on the capacitor since it has to now supply most of the current flowing through the motor. The motor draws current until the voltage in the circuit is able to provide minimum Vbe to keep the transistors on or able to overcome the motor resistance. Now, the voltage in the capacitor again rises until it reaches the trigger voltage of the trigger element and the above cycle repeats. This 1381 based SE is designed to increase the hysteresis width from a mere Vhyst to Vtrip-0.6V (0.6 V -being the minimum transistor turn on voltage).
Circuit modifications: Use 1381 of higher trigger voltage to further increase the hysteresis width would give longer running time but at less frequent rates. What happens if we vary the value of resistor? If you have a look at the circuit, you can figure out that the current through motor is because of Ic of NPN + Ib of PNP. So, increasing the value resistor will decrease Ib which will decrease motor current. So, the capacitor will take more time to discharge thus giving longer runtime but with decreased starting power. Similarly, decreasing R would increase Ib which will increase motor current. So, the capacitor will discharge faster and giving shorter running time with high starting power.
There is one more variant of 1381 based SE
Modified 1381 based SE:
Working:
This is an effective voltage trigger solar engine with configurable turn-onvoltage and discharge time.The turn-on voltage is determined by the type of 1381 selected & discharge time is determined by the timer capacitor C2. Here, the hysteresis width is not increased to a large value but it is tricked to take some time to come down from Vth + Vhyst to Vth, thus giving longer runtime of motor. While the solar cell charges the storage capacitor C1, the 1381 monitors the capacitor C2 which is also being charged through diode D1. At this point, the output (pin no.1) of 1381 stays low until the capacitor voltage does not reach Vtrip= Vth (2.2V) + Vhyst (of 50mV-300mV). Now, the capacitor voltage reaches the Vtrip & the output of 1381 switches to high state thus turning on the NPN device. The current then flows out of the storage capacitor, through the motor, through the transistor, and back to the other side of the capacitor. The 1381 output stays in high state as long as its voltage is above Vth. Once the storage capacitor voltage goes below Vth, the circuit goes in off condition. Suppose there was no C2 & diode in circuit then 1381 would have been watching the storage capacitor C1, it would reset as soon as the power dipped below Vth (as there is no PNP to keep the motor running and increasing the hysteresis width). But, here 1381 watches the timer capacitor C2 which is blocked from the power part of the circuit by the diode. So, it discharges out through 1381 to transistor. The discharging current is at operating current of 1381 which is of micro amperes, thus giving longer runtime of motor.
Circuit Modifications: The first thing you can vary is the 1381 trigger point, which will vary the turn on time of the motor i.e. varying the trigger point would vary the burst time of the motor giving longer bursts at lesser frequent rates for increasing 1381 trigger point v/s shorter bursts at higher frequent rates for decreasing 1381 trigger point. The next thing we can vary is the timer capacitor C2. Increasing the timer capacitor will increase the recharge time with longer bursts of motor runs and decreasing C2 will decrease the recharge time with shorter bursts of motor runs. Don’t increase the value of C2 so high that it keeps 1381 on even after the storage capacitor is fully discharged. The next thing you can vary is storage capacitor C1. Suppose we double the value of C1. This will make the motor run two times longer with double time to charge. To take full advantage of increased C1, increase the value of C2 to increase the runtime of motor.
Try building this effective solar engine for your solar rollers to give you better results than all previous solar engines.
The 1381 trigger ic & high storage capacitors: