Forum Diagram

The EPE Minder consists of two type- approved transmitter units and a receiver. If either transmitter becomes separated from the receiver, a buzzer in the latter part will sound.
The receiver is fitted with a switch to allow the use of only one transmitter if required.

MIND HOW YOU GO

This system was originally designed as a two-channel child alarm (to protect either a single child or two children at the same time) but many other applications spring to mind. For example, one transmitter could be placed inside a briefcase and another in a coat pocket. If the user forgot to pick up either of these items and walked away, the buzzer would sound in the receiver. The receiver must be carried on the per- son in a way that would make it practically impossible to lose it. This could be done using a belt clip, for example. Note that it will not be possible to use this system if either the transmitter or receiver were placed inside metal containers or if there were substantial metallic “screening” objects between them.

OPERATING RANGE
The operating range may be adjusted according to the intended purpose. However, it does depend on conditions. Adjustment is carried out by means of “aerial link wires” on the circuit panels. With all these in place, the range of the prototype exceeds 12 metres in open air. It will also work throughout several rooms indoors if required. If the battery voltage in either transmit- ter or receiver falls below a certain value, or if a transmitter is switched off, a buzzer will sound. The specified batteries in the transmitters should provide several hun- dred hours of operation. Those in the receiver should provide around 100 hours.

PERSONAL CODE
The EPE Minder uses a system of digitally encoded low-power radio signals,
which pass from the transmitters to the receiver. The code is different for each transmitter so that the receiver is able to distinguish one from the other. Type-approved, pre-aligned transmitter and receiver modules that operate at 433MHz. are used. No traditional “radio” skills are needed and no licence is needed for their use in the UK.

TRANSMITTER CIRCUIT
The circuit diagram for a single trans- mitter unit is shown in Fig.1. Current is
supplied to the circuit from a 3V “coin” cell, B1, via on-off switch S2 and diode D1. The diode provides reverse-polarity protection. It is best to use the specified Schottky device which introduces a smaller forward voltage drop, and therefore less loss, than a conventional silicon diode (0·2V rather than 0·7V approximately). Capacitor C2 provides a small reserve of energy and pre- vents the supply voltage from fluctuating. This stabilises operation. A low power 7555 timer, IC1, is set up in a standard astable (pulse generator) con- figuration. While switched on, this produces a continuous train of on-off pulses at its output, pin 3.The choice of resistors R1, R2 and capacitor C1 provide one pulse per second for one of the transmitters (Unit A) and one pulse every 1·2 seconds for the other one (Unit B). In fact, the timings are slightly longer but it helps to consider them as above. Also, the on times are much longer than the off ones in each case. The purpose of this will be explained presently.

RECEIVER CIRCUIT

Receiver module, IC1, requires a supply of between 4·5V and 5·5V. The 6V nomi-
nal battery pack, B1, is brought within range by the forward drop of diode D5
(0·7V approx.) This diode also provides reverse-polarity protection. Capacitor C4 charges up and provides a small reserve of energy. This will be useful when the battery is nearing the end of its operating life. When the supply voltage falls below some 4V, the receiver stops working and the buzzer will sound. Below around 3V, the buzzer itself will not operate so it is important to check operation each time the units are used. Receiver IC1 should be of the a.m. (amplitude modulation) type as specified in the components list. As such, it will respond to the on-off pulses provided by the transmitter. The inexpensive super regenerative (rather than superhet) variety will be perfectly adequate. The low-power variants of these receivers have not been tested. Although for battery operation they would appear to be ideal, the standard type is more readily available.

The receiver may be considered as hav- ing separate r.f. (radio frequency) and a.f. (audio frequency) sections. These have individual supply inputs (pins 1, 10, 12 and 15 with some being duplicated). These are all connected together and decoupled using capacitor C1.

TESTING

Having completed the Receiver board, we can now commence testing all three
boards. It helps to minimise the Receiver “hold-off” time by adjusting preset VR1 fully anti-clockwise (as viewed from the left-hand side of the p.c.b.) and preset VR2 fully clockwise (as viewed from the right- hand side of the p.c.b.). Check that the Test link has been left unconnected to prevent IC4b signal from passing to transistor TR1’s base. Switch on Single Channel switch S3 so that Channel A is enabled. With On-Off switch S4 off, insert the batteries. Switch on. After a short delay, the buzzer WD1 should sound. Now place Transmitter A approximately
three metres away from the Receiver, insert the battery and switch on. The buzzer should begin to bleep every second. The same procedure is now repeated for Transmitter B. To do this, switch S3 off to disable Channel A and firmly twist together the ends of the Test link wires. It is not advisable to solder this connection unless the i.c.s are removed first. The buzzer should bleep at a slightly slower rate than for Transmitter A. It is unlikely that the time periods of the two transmitters will be the same (due to overlapping component tolerances).
However, if they are, one of them will need to be changed. Choose slightly higher values for resistors R1 and R2 to slow it down and vice versa. Remove the i.c.s before making any modifications.

HOLD-OFF TIME
When both transmitters have been test- ed, switch S3 on to enable both channels. presets VR1 and VR2 should now be adjusted to approximately mid-track posi- tion. This should provide a sufficient “hold off” time plus a small margin. The buzzer should now remain off and only sound when one of the transmitters is switched off or moved out of range. Leave them operating for several minutes. If the occasional spurious bleep is heard, increase the settings of VR1/VR2 to pre- vent this happening.

The test beeper generates a sinusoidal signal with a frequency of 1,000 Hz, a common test frequency for audio amplifiers. It consists of a classical Wien-Bridge oscillator (also known as a Wien-Robinson oscillator). The network that determines the frequency consists here of a series connection of a resistor and capacitor (R1/C1) and a parallel connection (R2/C2), where the values of the resistors and capacitors are equal to each other. This network behaves, at the oscillator frequency (1 kHz in this case), as two pure resistors. The opamp (IC1) ensures that the attenuation of the network (3 times) is compensated for.

In principle a gain of 3 times should have been sufficient to sustain the oscillation, but that is in theory. Because of tolerances in the values, the amplification needs to be (automatically) adjusted. Instead of an intelligent amplitude controller we chose for a somewhat simpler solution. With P1, R3 and R4 you can adjust the gain to the point that oscillation takes place. The range of P1 (±10%) is large enough the cover the tolerance range. To sustain the oscillation, a gain of slightly more than 3 times is required, which would, however, cause the amplifier to clip (the ‘round-trip’ signal becomes increasingly larger, after all).

Circuit diagram:

To prevent this from happening, a resistor in series with two anti-parallel diodes (D1 and D2) are connected in parallel with the feedback (P1 and R3). If the voltage increases to the point that the threshold voltage of the diodes is exceeded, then these will slowly start to conduct. The consequence of this is that the total resistance of the feedback is reduced and with that also the amplitude of the signal. So D1 and D2 provide a stabilizing function. The distortion of this simple oscillator, after adjustment of P1 and an output voltage of 100 mV (P2 to maximum) is around 0,1%. You can adjust the amplitude of the output signal with P2 as required for the application. The circuit is powered from a 9-V battery. Because of the low current consumption of only 2 mA the circuit will provide many hours of service.