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NEW - Full details and plans for PIR Sensor Circuits are now available by clicking here. We also have complete PIR Relay Timers available now in the REUK Shop.

Weaknesses of PIR sensors
In order for a PIR sensor to work well most of the time, they are designed with certain limitations. A PIR sensor cannot detect a stationary or very slowly moving body - if the sensor was set to the required sensitivity, it would be activated by the cooling of a nearby wall in the evening, or by very small animals. Similarly, if someone walks straight towards a PIR sensor, it will not detect them until they are very close by.

PIR sensors are temperature sensitive - they work optimally at ambient air temperatures of around 15-20 degrees Celcius. If the temperature is over 30 degrees, the field of view narrows and the sensor will be less sensitive. Alternatively, if the temperature is below 15 degrees, the field of view widens and smaller or more distant objects will activate the sensor.
In a previous article we covered how the LM317T voltage regulator chip can be used to obtain a fixed voltage from a variable supply. Looking at a typical renewable energy system 12V battery bank, the output from the batteries can rise to 15V+ when they are being heavily charged. In order to power lower voltage devices from such a battery bank, some kind of voltage regulation is essential.

LM317 / LM317T
The LM317T is a very common voltage regulator (available from the REUK Shop) used to obtain a fixed voltage of between 3 and 40 Volts set using a couple of resistors. Click here to read up on using the LM317T to regulate voltage.

Fixed Voltage Regulation
While the LM317T is relatively simple to use, having to add a couple of resistors adds unnecessary complexity if you just want a fixed voltage of say 5 Volts at around 1 Amp - for example, for a simple solar battery charger powered from a 12V solar panel or 12V battery bank.

Permanently fixed voltage regulators are available which require no resistors - the output voltage is internally fixed. The L7800 Series of positive voltage regulators can handle an output current of up to 1.5 Amps, and regulators are available with output voltages of 5, 5.2, 6, 8, 8.5, 9, 12* (pictured above), 15, 18, and 24 Volts.
* A couple of volts are dropped (i.e. lost) in these regulators , so the 12V regulator cannot be used reliably with a 12V battery bank to obtain a fixed 12V ouput.

These fixed regulators are available cheaply (well under ?1 each) and are very simple to use in practice.

Using an L7806CV Voltage Regulator
The L7806CV is a voltage regulator from ST which will provide a fixed output of 6 volts (5.75-6.25V in reality) from an input voltage from 9V to 35V. Therefore, it it suitable for use for a 4 AA Battery Charger to be powered from a 12V solar panel or battery bank.
In even the most basic renewable energy system it is essential to monitor the charge in the battery bank, the voltage and current coming from the generator, and the current being drawn by the power inverter or other loads. In this article we will cover moving coil meters - the cheapest and simplest devices for displaying current and voltage in a system.

Moving Coil Voltmeters
The simplest way of monitoring the voltage of the battery bank is a moving coil voltmeter. These are available from around ?5-10 each and can simply be wired to the positive and negative outputs of the battery bank. They do not need their own battery power since they take a tiny current from the batteries.

Pictured above is a 8-16 Volt DC Analogue Voltmeter which is perfect for measuring the voltage of a 12V battery or 12V battery bank.
NEW We have an excellent top-quality 0-30VDC moving coil voltmeter for sale in the REUK Shop.

Moving Coil Ammeters
Ammeters, used to measure the current flowing into or out of the battery bank, are also available for a wide range of different applications from micro-Amps (e.g. in solar cell experimentation) to hundreds of Amps (e.g. in large wind turbines). Pictured below is a 0-20A moving coil ammeter.

Since the current flows into and out of the battery bank as the batteries are charged or discharged, the most suitable renewable energy system ammeters are centred on zero - a positive reading indicates that the batteries are being charged by the generator, and a negative reading indicates the batteries are being discharged.

Above is a photograph of an automotive analogue ammeter which shows currents from -200 to +200 Amps. This would have limited use in most domestic renewable energy systems except with very large wind turbine generators and multi-kW hydroelectric generators since the current range is so large - however there are many smaller range ammeters available - e.g. -20A to +20A which would be perfect for a small domestic wind turbine generator or solar installation.

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6th ave electronic

The supply current, when the output is 'high', is typically 1 milli-amp (mA) or less. The initial monostable timing accuracy is typically within 1% of its calculated value, and exhibits negligible (0.1%/V) drift with supply voltage. Thus long-term supply variations can be ignored, and the temperature variation is only 50ppm/°C (0.005%/°C).

All IC timers rely upon an external capacitor to determine the off-on time intervals of the output pulses. As you recall from your study of basic electronics, it takes a finite period of time for a capacitor (C) to charge or discharge through a resistor (R). Those times are clearly defined and can be calculated given the values of resistance and capacitance.
The basic RC charging circuit is shown in fig. 4. Assume that the capacitor is initially discharged. When the switch is closed, the capacitor begins to charge through the resistor. The voltage across the capacitor rises from zero up to the value of the applied DC voltage. The charge curve for the circuit is shown in fig. 6. The time that it takes for the capacitor to charge to 63.7% of the applied voltage is known as the time constant (t). That time can be calculated with the simple expression.

Pin 1 (Ground): The ground (or common) pin is the most-negative supply potential of the device, which is normally connected to circuit common (ground) when operated from positive supply voltages.

Pin 2 (Trigger): This pin is the input to the lower comparator and is used to set the latch, which in turn causes the output to go high. This is the beginning of the timing sequence in monostable operation. Triggering is accomplished by taking the pin from above to below a voltage level of 1/3 V+ (or, in general, one-half the voltage appearing at pin 5). The action of the trigger input is level-sensitive, allowing slow rate-of-change waveforms, as well as pulses, to be used as trigger sources. The trigger pulse must be of shorter duration than the time interval determined by the external R and C. If this pin is held low longer than that, the output will remain high until the trigger input is driven high again. One precaution that should be observed with the trigger input signal is that it must not remain lower than 1/3 V+ for a period of time longer than the timing cycle. If this is allowed to happen, the timer will re-trigger itself upon termination of the first output pulse. Thus, when the timer is driven in the monostable mode with input pulses longer than the desired output pulse width, the input trigger should effectively be shortened by differentiation. The minimum-allowable pulse width for triggering is somewhat dependent upon pulse level, but in general if it is greater than the 1uS (micro-Second), triggering will be reliable. A second precaution with respect to the trigger input concerns storage time in the lower comparator. This portion of the circuit can exhibit normal turn-off delays of several microseconds after triggering; that is, the latch can still have a trigger input for this period of time after the trigger pulse. In practice, this means the minimum monostable output pulse width should be in the order of 10uS to prevent possible double triggering due to this effect. The voltage range that can safely be applied to the trigger pin is between V+ and ground. A dc current, termed the trigger current, must also flow from this terminal into the external circuit. This current is typically 500nA (nano-amp) and will define the upper limit of resistance allowable from pin 2 to ground. For an astable configuration operating at V+ = 5 volts, this resistance is 3 Mega-ohm; it can be greater for higher V+ levels.

Pin 3 (Output): The output of the 6th ave electronic comes from a high-current totem-pole stage made up of transistors Q20 - Q24. Transistors Q21 and Q22 provide drive for source-type loads, and their Darlington connection provides a high-state output voltage about 1.7 volts less than the V+ supply level used. Transistor Q24 provides current-sinking capability for low-state loads referred to V+ (such as typical TTL inputs). Transistor Q24 has a low saturation voltage, which allows it to interface directly, with good noise margin, when driving current-sinking logic. Exact output saturation levels vary markedly with supply voltage, however, for both high and low states. At a V+ of 5 volts, for instance, the low state Vce(sat) is typically 0.25 volts at 5 mA. Operating at 15 volts, however, it can sink 200mA if an output-low voltage level of 2 volts is allowable (power dissipation should be considered in such a case, of course). High-state level is typically 3.3 volts at V+ = 5 volts; 13.3 volts at V+ = 15 volts. Both the rise and fall times of the output waveform of 6th ave electronic are quite fast, typical switching times being 100nS. The state of the output pin will always reflect the inverse of the logic state of the latch, and this fact may be seen by examining Fig. 3. Since the latch itself is not directly accessible, this relationship may be best explained in terms of latch-input trigger conditions. To trigger the output to a high condition, the trigger input is momentarily taken from a higher to a lower level. [see "Pin 2 - Trigger"]. This causes the latch to be set and the output to go high. Actuation of the lower comparator is the only manner in which the output can be placed in the high state. The output can be returned to a low state by causing the threshold to go from a lower to a higher level [see "Pin 6 - Threshold"], which resets the latch. The output can also be made to go low by taking the reset to a low state near ground [see "Pin 4 - Reset"]. The output voltage available at this pin is approximately equal to the Vcc applied to pin 8 minus 1.7V.

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