Here is my current thought on 1S altimeter ignition using 3.3V TTL logic. VBat is a 1S Lipo which ranges from 4.2V-3.2V.
When the low impedance FIRE1 pin is pulled high, the NPN (100V 5A) transistor allows current to flow between VBat and GND. The 1K resistor on the base of Q1 provides ~3.3ma which given the gain of the Darlington will allow ~3.3A of current to flow through the ematch at ~(VBat - .3V) so should provide plenty of kick for the ematch (The ones I use work fine at 2V, 2.5A)
When there is continuity across Ematch1, the opto is powered through the 1K resistor. The opto drops about 1.3V, so we have ~2mA through the ematch all the time. When the opto turns on, current can flow from VBat through the 1K, opto and schottky diode to the CONT1 digital input which has high impedance(~1MOhm) The 1K is not really necessary, just there in case some bounce on startup sets the digital CONT pin to low resistance. The idea is that the NPN in the opto and the schottky give about .4V CE drop + .5V Schottky diode drop, which drops the voltage of the Battery by 0.9V. Fully charged the battery is 4.2V - .9 = 3.3V, discharged the battery is 3.2V - .9 = 2.3V both of which are within the range of 3.3V TTL high.
Finally, when the charge fires, the opto shuts off, but the capacitor C10 holds the CONT1 input high for about 5s +- based on the time constant.
Comments: I had thought about doing this with a P-Channel mosfet, but was concerned about firing on power up as the pins float down at power up for some 10s of milliseconds.
A couple of things I have noted:
1. The use of the opto seems silly here, since I have tied the same voltage rail to both ends! I had first tied the NPN of the opto to the 3.3V rail which made sense but then started to worry that the voltage drops across the diode and the NPN would pull the voltage below the TTL high threshold.
2. It seems like I could eliminate the opto and do this:
3. I thought about doing a voltage divider, but then the diode isn't there to block the cap discharge which holds the continuity input high.
4. I think the TIP122 equivalent is overkill and would like to go do to a smaller SOT223 NPN part which will do 2A.
Any thought on favorite ways to do this?
When the low impedance FIRE1 pin is pulled high, the NPN (100V 5A) transistor allows current to flow between VBat and GND. The 1K resistor on the base of Q1 provides ~3.3ma which given the gain of the Darlington will allow ~3.3A of current to flow through the ematch at ~(VBat - .3V) so should provide plenty of kick for the ematch (The ones I use work fine at 2V, 2.5A)
When there is continuity across Ematch1, the opto is powered through the 1K resistor. The opto drops about 1.3V, so we have ~2mA through the ematch all the time. When the opto turns on, current can flow from VBat through the 1K, opto and schottky diode to the CONT1 digital input which has high impedance(~1MOhm) The 1K is not really necessary, just there in case some bounce on startup sets the digital CONT pin to low resistance. The idea is that the NPN in the opto and the schottky give about .4V CE drop + .5V Schottky diode drop, which drops the voltage of the Battery by 0.9V. Fully charged the battery is 4.2V - .9 = 3.3V, discharged the battery is 3.2V - .9 = 2.3V both of which are within the range of 3.3V TTL high.
Finally, when the charge fires, the opto shuts off, but the capacitor C10 holds the CONT1 input high for about 5s +- based on the time constant.
Comments: I had thought about doing this with a P-Channel mosfet, but was concerned about firing on power up as the pins float down at power up for some 10s of milliseconds.
A couple of things I have noted:
1. The use of the opto seems silly here, since I have tied the same voltage rail to both ends! I had first tied the NPN of the opto to the 3.3V rail which made sense but then started to worry that the voltage drops across the diode and the NPN would pull the voltage below the TTL high threshold.
2. It seems like I could eliminate the opto and do this:
3. I thought about doing a voltage divider, but then the diode isn't there to block the cap discharge which holds the continuity input high.
4. I think the TIP122 equivalent is overkill and would like to go do to a smaller SOT223 NPN part which will do 2A.
Any thought on favorite ways to do this?