Bill of Materials: General Purpose Alarm, Pulsed PART NO

Transcription

1 General Purpose Alarm, Pulsed PART NO I hate alarms that sound continuously - unless they are smoke alarms. Smoke alarms should be annoying, but others should not. I wanted an alarm for a function in my home, but I wanted it to pulse on and off at about a 1 per second rate. So I took an LM556 chip (a dual timer) and used one side as an oscillator to generate a 2kHz audio tone and the other side to generate a pulsing signal of about 1 Hz to control the oscillator. Powered it with 9 volts, ran it to a small speaker (2" dia), and configured it so it could be activated either with a switch closure or with a small NPN transistor (such as a PN2222). Mounted it all on a small printed circuit board (PCB) - and there you have it. With a 9V supply it is fairly loud and very effective, and it will function down to about 2.5V although the tone is very soft at that supply voltage. When activated, it draws 104 ma from the supply at 9V and 15 ma from the supply at 2.5V. When not actively sounding, it draws no current at all. The kit includes most of the parts you need, including the PCB (with mounting holes ), the chip, the "silence alarm" switch, and the speaker. A 9V battery is supplied, but you may use another power source. You will get some hook-up wire for jumpers and mounting the speaker, and your switch or other circuit for triggering it. This gives you a handy audio alarm module to use any way you need to... Time Required: 2 hours depending on experience Experience Level: Beginner Required tools and parts: Soldering pencil (small, sharp point) Rosin core solder (small diameter) De-soldering wick (optional, but may be very helpful to remove solder bridges) Small needle nose pliers Small wire cutters Magnifying glass (very helpful for inspecting your work, but optional) 22 ga insulated hook-up wire (for jumpers) 20 ga insulated hook-up wire (for mounting the speaker) Bill of Materials: Qty Jameco SKU Component Name K ohm, 1/4W resistor K 1/4W 5% Resistor k ohm, 1/4W, 5% resistor uf 16V electrolytic capacitor uf, 100V mylar capacitor LM556 Dual Timer chip PN2222A Transistor Ohm, 2.9" dia, speaker V Battery Connector - 4" Leads SPDT toggle switch, right angle PCB mount Step 1 - General Soldering Guidelines 1. Installing components in the order suggested in these instructions will result in less risk of physical interference between

2 components mounted earlier and those mounted later. 2. As a general rule, anytime you have inserted "the last wire or pin" that will be going into a particular pad on the PCB, you should go ahead and make the solder connections to those wires and pins on that pad. It is easier to wait until you can make all solder connections to all wires and pins on a particular pad, rather than soldering them one by one as you add them. Just one solder bond to the pad, involving all wires and pins is the best. Soldering these as you go along, will help prevent wires and components moving or falling out of the board as you move the board around to install later components. 3. Once you insert a wire or component pin through its appropriate hole in the PCB, splay out the wire or pin just a little bit to help it make contact with the copper at the edge of the hole through the board. Then trim any excess wire you may have - leaving no more than about 1/8" - before soldering. 4. Have a nice, sharp tip on your soldering pencil and things will be easier. Having some de-soldering wick available may be helpful. A magnifying glass is handy for examing connections and possible solder bridges. 5. Viewing the bottom (foil) side of the board with a magnifying glass - from the bottom with a light on the other side - is the easiest way to check that you have no solder bridges between foil runs. Step 2 - Theory of Operation This won't help you assemble this module, but it will help you understand how it works... (optional) Refer to the Schematic Diagram. The LM556 is a dual timer chip giving us two independent LM555 timers in one 14-pin DIP package. We use the first half of the chip (the right side of the schematic diagram - U1b) to implement an oscillator at about 2 khz. The output comes out of pin 5 and goes to the speaker through a capacitor. The resistors and the 0.01 uf mylar capacitor give us the frequency we want. The oscillator will only work when the lower end of C4 is connected to ground. Since we want to "pulse" this thing, we insert Q1 at that point so when we turn it ON, it brings C4 down to ground potential and the oscillator turns on. When we have Q1 OFF, the oscillator is silent. We control the state of Q1 with R1 which is driven by the pin 9 output of the other section of the timer. The other section of the timer (U1a) is an oscillator that pulses the "audio oscillator" section at about a 1 Hz rate and with a pleasing pulse width duty cycle. This is controlled by the resistors and C1. To control this thing we have the "Trigger", or sensor, in series with a "silence" switch. To "arm" the alarm, one sets the Silence switch to ON. Then when a closed circuit occurs across the "sense" wires the alarm will sound. One can then silence the alarm with the switch (while you work - with the alarm no longer going off in your face - to clear the condition that initiated the alarm in the first place). The "trigger sensor" can be a mechanical switch or something electronic like a transistor or an FET. The currrent that will need to pass through this device is the current draw shown in table in the Operating Conditions step (max of about 104 ma if operating at 9 Vdc). This current can easily be handled by a microswitch or a small transistor.

3 Step 3 - PCB Layout Diagram Refer to this diagram to see where particular components are placed on the PCB. (In some cases additional detail will also be provided later.) Step 4 - Insert the Timer Chip U1 1. Insert the LM556 chip at its location on the board, ensuring that pin 1 is at the upper left. 2. Carefully "jostle" the pins slightly, if needed, to get them all to go through their respective holes in the PCB using a small sharp tool. 3. Don't solder the connections yet. 4. Note there are two pins going into small pads that make no connections to other wires or components. Since the other pins that will be soldered later will provide more than enough mechanical strength to hold the chip adequately, you do not ever need to solder these two pins that have no other connections to their small pads on the board. Step 5 - Install the 5 Jumper Wires Following the pictorial diagram, cut and install the 5 jumper wires in their positions on the board. Don't solder them yet. Insulated #22 AWG is the largest wire gauge you should use to have adquate physical space available on the board. You can also use #24 or smaller wire if you wish (all these pass very small currents so small wire gauges are OK).

4 Step 6 - Install the Transistor Install the PN2222 transistor, Q1 at the position shown in the diagram. Don't try to push the device down too close to the PCB - having the body of the transistor about 3/16" above the board is about ideal so there is enough "lead" to avoid overheating the device when soldering. You should also have the 3 jumper wires that connect to the transistor leads installed at this point, so go ahead and make the solder connections for these three transistor pads. Allow at least about 15 seconds between soldering each pad to try to limit the temperature the transistor internals reach when you heat the connections. Step 7 - Install the Capacitors Place the four capacitors as shown in the pictorial diagram, and solder them in place at this time. You can refer to the overall PCB pictorial diagram. The photo here of the capacitors installed, and also the diagram that shows just the capacitor placement to help you. The three large electolytics are all of the same value so they can go in any of the electrolytic (polarized) positions. Just be sure to install the polarized pins in the orientation shown in the diagram. Note that you are installing the "legs" of the small 0.01 uf mylar capacitor "over" a jumper wire. Be sure that the jumper wire goes "cleanly" between the two legs of the capacitor without touching either so there will be no compromise of the wire insulation when you solder the capacitor in place and those leads get hot a little later.

5 Step 8 - Install the Resistors Refer to the overall PCB layout diagram to identify the spots where the resistors are placed, and refer to the Resistor Diagram to see where the particular resistors (particular values) should be placed. You can also use the photo here to help you identify where they go and what they look like. Note: After all these photos were taken it became necessary to substitute the speaker with a different type. The magnet on this replacement speaker is larger than the previous one, and to reduce potential mechanical interference problems later (when you install the speaker), it is best if you install the 33K resistor that is to the right and a little below the IC differently than shown in this photo. Keep the resistor and the leads low down to the PCB instead of elevated as shown in the photo. Step 9 - Install the Switch 1. There are two small metal tabs up near the toggle handle on this switch that are troublesome. We don't need them to attach to the board, and they will just poke your fingers when you grab the switch to use it later, so just fold them back against the body of the switch to get them out of the way. 2. Insert the switch as shown in the diagram and solder the two lower pins (the top one is still waiting for a wire that we will mount next).

6 Step 10 - Install the Battery Connector and Sensor Wires 1. Install the battery connector in the top two holes in the upper right of the PCB as shown in the photo. The red wire (+) goes to the top hole. 2. Install the "sense wires" in the two PCB holes below the battery connector as shown in the photo. These should just be stub wires, of perhaps 6" long, that you will use to connect to your "alarm trigger sensor" (whatever you are using) when you install this board. The wire coming from the pad near the switch should be red (to indicate +) and the other wire, above it, should be black (to indicate -). You can also just directly attach the wires coming from your trigger if you wish, but having these stubs is usually desirable for convenience later. 3. Note that the alarm is "triggered" by a "switch closure" across the "sense wires" you just installed above. If you are using a mechanical switch of some sort, the polarity of the connection to the wires doesn't matter. But if you are using a transistor or FET to trigger the alarm, which you can because the current draw is very low, then be sure to connect the red (+) wire to the collector of the transistor and the black (-) wire to the emitter. Step 11 - Prepare the Speaker for Mounting 1. Cut two pieces of #20 AWG hookup wire about 3/4" long. One end of these will be soldered to the speaker terminals, and the other ends will go into the PCB. 2. You don't need any insulation at all on these, so remove it. Install the speaker by inserting these two wires into and thru the PCB and getting the speaker as close to the board as you can without contacting any of the other components.

7 Step 12 - Mount the Speaker ** "We have needed to substitute a slightly larger speaker than the one that was used in the original design (due to non-availability of the original part), so you may need to modify the physical mounting scheme for the speaker to suit your purposes." ** Now insert the two wires from the speaker into the PCB at the position shown (the last two small holes in the PCB that are available) and push them in far enough that the speaker can be positioned perpendicular to the board and getting the speaker as close to the board as you can without contacting any of the other components. The wires are fairly stiff, but the closer you can get the speaker to the board the more robust the final module will be. You can also use a bit of electrical tape or a small piece of cardboard to provide insulation between the speaker and other components if you wish (I did not do any of this for the photo). Solder the two wires to the board. Note that you should have installed the 33K ohm resistor "lower to the board" than shown in this photo - where one of its leads is coming close to the speaker housing. Due to the change in speaker after the project was built, it is better to have this resistor "low to the board" rather than elevated as shown in this photo. Step 13 - Final QC Check Now make one last inspection of your work. Ensure that you have soldered each pad, wire, and component. Try to find any solder bridges between pads and repair them if found. Once you think you are ready, attach a 9V battery and close the connection between the sense wires. You should hear a pulsing alarm tone of good volume at about 2kHz frequency. Congratulations! Step 14 - Operating Conditions The figure shows a table of the current draw that was observed with various applied DC voltages. The alarm operates (but very faintly) down to 2.5 Volts, but you would probably never use it under these conditions. The current draw increases as shown up to the max intended voltage of 9 Volts, at which point the alarm is "comfortably loud".